Originally, it looked like the extension template loader was working just fine.  I was wrong - for some reason, attempting to load an extension template from an extension that had *just* been enabled (and had been disabled at server boot) caused an exception (a "NoneType" was being called - eventually, I tracked this down to the get_provider function in pkg_resources... importing resource_string from pkg_resources didn't seem to do the job of initializing that function).  Importing the _manager directly seems to fix this.

I also realized there was a templatetag caching problem occurring.  I've solved this by clearing the templatetag cache upon extension enable/disable.

Manual.