Chord Pianissimo is a tiny Japanese website which has been updating since January with wonderful piano arranged tracks from several Cave shooters. The guy started with Ketsui, but now he seems to be moving on to Mushi Futari.
So far he has done the following three updates (note that the dates are in the Japanese format: yy/mm/dd):
07/07/01
Mushihime-sama Futari Stage1 Tabitachi no Futari
http://www.geocities.jp/fh_x4/
07/04/06
Ketsui ~Kizuna Jigoku Tachi~ Stage 5 EVAC INDUSTRY
http://www.geocities.jp/fh_x4/002_ketsui/index.html
06/01/22
Ketsui ~Kizuna Jigoku Tachi~ Stage 1 INTERCEPTION
http://www.geocities.jp/fh_x4/001_ketsui/index.html
To get the mp3 files follow the links and look at the top left corner of each page. If you enjoy them (the Ketsui Stage 5 track is a must-listen), I suggest you send a brief email to the guy (fh_x4@yahoo.co.jp) letting him know. Just a few words in English will do.
If he gets enough emails there's more of a chance he'll come up with more tracks in the future...