Returning proxies from association extensions?

Has anyone tried creating proxies in their association extensions?

For example, Jamis talks about extensions here - http://
There's an 'active' method, which returns the results of a find
operation. But say I need to count those active items. I could add a
count_active method... but it seems like it would be much cleaner if
the active method returned an association proxy. That way, a simple could be queried for the full range of proxy
methods - eg, or

The association proxy setup code is making my head spin right now, so
I haven't tried it - has anyone else?

That's a no, then?