I am using less and coffee in one of my projects. I found Steve Losh's extension which does the less compilation https://github.com/sjl/flask-lesscss/blob/master/flaskext/lesscss.py and I read somewhere there is another based on it for coffeescript.
The extension, though quite simple, does a os.walk for each request irrespective of what the request does.
I am using a slightly modified version which doesn't do an os.walk and handles blueprints.
EDIT: I can't figure out how to paste code.
This snippet by Rahul Kumar can be used freely for anything you like. Consider it public domain.