I put together a quick and moderately dirty hack to modify TextMate's Info.plist so that I get Quick Look support for text types that it claims on Leopard. Source code only, use at your own risk (see comments at the top for usage). As far as I understand, using UTImportedTypeDeclarations should not override the more correct UTIs supplied by the system or other apps, but I won't guarantee that.
http://homepage.mac.com/amaxwell/.cv/amaxwell/Sites/.Public/utconvert.m-zip....