I know that Metacity is no longer getting new features, but I wonder if some of the non-Clutter additions to Mutter (like the new extensions) were pushed to Metacity? If so, that might explain why your Mesa stack can't run it. Have you tried installing Mutter? I'd be interested to see if that composites for you.
I'm sure I asked this before but I take it you queried gconf to verify it was on?
gconftool-2 -g /apps/metacity/general/compositing_manager
should be true.