Since there is not much documentation on this yet, here are some notes:
OS/X HINT: use absolute filenames for input, otherwise the project file will be found but not the referenced files
Fileformat:
infile1
infile2 <TAB> outfile
<CONFIG
OUTFMT [copy this part from reaper-render.ini]
>
Example:
/Users/Hank/Documents/REAPER Media/HitRecord.RPP HitRecord.mp3
/Users/Hank/Documents/REAPER Media/AnotherHitRecord.RPP AnotherHitRecord.mp3
<CONFIG
<OUTFMT mp3 44100 2 0 1 3 0
bDNwbSAAAAABAAAABQAAAAAAAAADAAAAQAEAAAAAAAA=
>
>