Yep, that would be a nice one to have! :-) I've done it myself for textile (using a jar from the mylin wikitext project), but it appears as markdown has more momentum going for it than textile has for the moment.
I just got a patch from another user that added a markdownToHtml method to the string class (I just released the 0.2 version of the plugin with this).
I've looked a little bit at the encoding stuff in grails and it feels a little clunky. I think the method added to string gives the same functionality and is a little more straightforward in it's implementation. I'm going to close this issue with the addition of that functionality.
If there's some additional use that the encoding stuff gives that I'm missing, feel free to reopen, but I think the basic requested functionality is covered.