I'm looking for some willing participants to record some rhythm tracks (any style, bpm), it's only a test so don't worry about them being perfect, and any style will do.
If it works out ok then it will be a way users can share and collab their instruments and drums.

Here's a basic recording template (you don't need ReaTrak installed to record them)
I made 64 bars of verse and 64 of chorus, it includes a drum track with no fills, record the verse in one style and chorus in another,
it has one ending for verse and chorus. If you like just record every second semitone then just transpose the recorded audio.
You can create one with any progression you like, just try and cover the basic chords.
These are only basic for you to try, you can create more advanced ones with fills so whatever you play in the fill will be matched, you can also link sections with -L1 -L2 -L3 etc. this will match these section to the user's progression. You can create more variety of endings, you can play some pickup/intro notes in the count-in(s). "Transpose Chords Selection" in ReaTrak will transpose the regions in the time selection by semitones.

If you have ReaTrak installed you can add other backing to record to by selecting some guitars/piano and bass from the

If you record DI Direct Input you will get better results without the sustain bleeding into the next bar, FX can be added after it has been fitted to the user's chord progression.

If you want to try them out read below else just upload them to dropbox or mediafire free accounts.
Save the regions.csv in the region manager or "Export\Renumber Chords" button (this will also snap chords to the grid in case they are out) and rename to same name as the recorded wav.
Put your instrument tracks in a folder with name format eg.
X:\ReaTrak Instruments\Guitar, Electric, Rhythm Classic Rock 01 Ev8 4-4 120bpm
Guitar, Electric, Rhythm Classic Rock 01 Ev8 4-4 120bpm C-Am.csv
Guitar, Electric, Rhythm Classic Rock 01 Ev8 4-4 120bpm C-Am.wav

You need this in the name format:

4-4 is time signature
120bpm is tempo
C-Am is Key

To test them out
enter your chord progression and color the regions for the song part,
select all regions with the time selection,
click Choose Instrument button and choose the wav you recorded,
click ReaTrak Selection,
it should fit the instrument to your chords.
If it's all good you can upload and share/collab.
ReaTrakStudio Chord Track for Reaper forum