I just discovered that it exists: a free synth called REAPER MADNESS. And yes, it works fine in REAPER, but it's only a must have for people who are raving mad about REAPER and those who expect to make a score once for a sleazy B-movie with loads of ketchup and a whole artillery of chainsaws.
Download at
http://onlydover.org/ (see the synths list)
The looks of the thing are a fine indication of what you get:
https://stash.reaper.fm/oldsb/1150675...perMadness.png