There already exists a Tcl language syntax. It's just not bundled with TextMate. Check out the subversion repository to get it: http://macromates.com/wiki/pmwiki?n=Main.SubversionCheckout
Note that a svn checkout means you must keep it updated. Easier nowdays to just have the application since all the betas now include the default bundles. You can checkout just the specific bundle you need from svn however, a perfect solution.