Saskatoon, a code editor forked from Smultron, had a built-in Textile pipe. It was discontinued but fortunately the source code is still available on its website. You or anyone else might want to check it out if it helps you port that function over to TextMate.
Saskatoon http://saskatoon.sourceforge.net Smultron http://smultron.sourceforge.net
Paul Bissex, the creator of Saskatoon, might be able to help you out as well - http://e-scribe.com
PS thanks whoever did Textile!
Dean Allen is the man. http://www.textism.com