What follows is my recipe for rewiring Reaper as a slave in Samplitude.  This is designed to be used with a hardware module, as distinct from using the "route 16 channels of MIDi to this track," which is a feature in Reaper that only works with a software instrument.  

Rewiring Reaper as a multi timbral slave to Samplitude
Reading this post assumes you are familiar with Reaper and that you have also read and digested my post on how to create a 16 channel multitimbral track layout using ReaMIDI Control within Reaper.  To expand on the recipe for that, you need to use Gianluca’s guidance on how to route the tracks in the multitimbral Reaper instance for Rewire:
A.  Press Alt and Applications on each track and uncheck master send.
B.  Press Alt and Applications a second time and route the hardware output to a separate Rewire out.  This may not be necessary because, in contravention of Gianluca’s other instructions, it doesn’t appear necessary to choose multiple outputs as the option in Samplitude.  Instead, it appears that one is best served to enter Rewire as an effect with audio and MIDi on same track exactly the way you would with any other VST in Samplitude.  The fun comes later.  
Assuming that you have 16 MIDi tracks in your Samplitude project, do the following:t

1.  Go to track 1 and press CTRL Windows grave accent to bring up the insert menu.
2.  Choose Rewire, second from the bottom and right arrow to choose Reaper as your Rewire effect.  
3.  When Reaper comes up, you should insert the 16 channel track template you made according to the instructions above.  If you didn’t make such a track template, BAD YOU!
4.  When your track template loads, move to track 1 of the template and arm it.  Make sure that input monitor is set to Normal.
5.  Return to Samplitude and go to Track 1 of your project where you have instantiated Rewire.  Press Alt I to bring up the Track Properties dialog.  Make sure that your MIDi Record device is set to ALL and your MIDi Playback Device is set to Reaper.  
6.  Tab to the MIDi IN and make sure that is set to ALL while your MIDi OUT is set to 1.
7.  To make sure that things are working best, it is recommended that you enable the Automatic MIDi Record Switch on Current Track in Samplitude Just because.  
8.  Having left the Transport Console, arm track 1 and make your first recording.  
9.  When this track is done, go into the Track Properties dialog and change your MIDI playback device to whatever your hardware is that you are using in Reaper to play your 16 channels.  
10.  Alt TAB to Reaper and unarm track 1 in your template and turn monitoring OFF.  You should not hear anything when you play this track on your keyboard.  But you should hear something when you play the project back.  If this is not the case, you have problems.  
11.  Go to the second track in your project and instantiate Rewire as shown in step 1 above.  
12.  This time, go to track 2 on your Reaper template and arm and monitor it.  
13.  You should have a different bank and patch chosen for this track so that you can distinguish it from the track you just recorded.  
14.  Return to Samplitude and, in the Track Properties Dialog, make sure that the MIDI Playback Device is set to Reaper, initially, and that the MIDI IN is set to All while the OUT is set to 2.  
15.  Having armed both the second track in the Reaper template and the second track in your Samplitude project, record your second track.
16.  If you are hearing two patches at once while you are playing your MIDI keyboard, go back to Track Properties in Samplitude and change the MIDi Playback Device from Reaper to your MIDI keyboard while still keeping the IN channel set to All and the OUT channel set to 2, respectively.  Alternatively, you can simply make sure that channel 2 is the only one armed and monitored on the Reaper side.       
17.  Once you’re satisfied with this track, go back to the Track Properties in Samplitude and reset your MIDi Playback Device to your hardware if you didn’t do so in step 16.    

Continue with as many tracks as you want in your 16 tracks taking care to follow all the steps above as appropriate with each one. 

Here’s a follow-up to my earlier e-mail with the rewire recipe.  I’ve learned some seemingly fascinating things.
1.  Remember that I said you had to set the MIDi Playback Device to Reaper and the MIDi Out channel in Samplitude to whatever the specific number was when recording.  To get multi-timbral separation after recording, you had to reset the MIDi Playback Device to your hardware.  I think I’ve now learned that you can avoid having to reset the MIDi Playback Device as long as you only arm and monitor the specific channel in Reaper on which you’re playing.  In that case, the MIDi Playback Device can remain Reaper.  

2.  Equally fascinating is the fact that, as I may have pointed out earlier, when you reopen the Samplitude project, the Rewire device is not engaged by default.  There are no effects in any tracks.  You can confirm this by pressing CTRL Windows 1.  You will hear JAWS say, “no effects.”  

3.  Here’s the fascinating part.  Whatever tracks you have already recorded in your project will play back with the hardware sounds that you had to utilize through rewire, even though there is no instance of rewire available.  This is provided that you reset your playback device to your MIDi hardware before saving and closing the project.    
4.  But, if you want to add any more MIDi tracks utilizing the rewired instrument, you have to reinstantiate rewire on the track on which you wish to record, change its MIDi OUT to the correct number and the playback device to Reaper before arming the Rewire track in Reaper and the recording track in Samplitude and laying down your track.  Then, as before, change the playback device to your hardware.  

5.  If you then save the file, you will find that, upon opening the file again, any tracks which had ever been assigned to Reaper instruments from your hardware synth will be assigned to those instruments by default and you can rerecord anything you want on those particular tracks.  You won’t be able to change patches on those tracks without reinstantiating Reaper on at least one track in the project and reloading the template you should have made initially to facilitate your choosing banks, channels and patches in your hardware instrument.  

Finally, this whole enterprise makes it unmistakably clear that Samplitude suffers terribly for lack of a track template facility.  It would be cool if one could create a track template that would give you 16 tracks of rewired stuff.  On the other hand, why is it that the use of rewire is not preserved when a project is saved?