Flexibility for fast_cache?

First of all, amazing work here Steve. Hats off.

Secondly, and more specifically, I love the inclusion of DRUPAL_BOOTSTRAP_EARLY_PAGE_CACHE support.

I'm working on designing some systems that use frequent xhttpd requests to maintain a live "state", and scaling them effectively beyond a single server means putting the state request into a memcache cloud. Currently I have my own interface to that as a standalone php file, but being able to run that off the EARLY_PAGE_CACHE should (in theory) let me keep that stuff inside Drupal and get about the same performance.

I can do this now, but I'm restricted to using the cache_page bin and having everything work off uris.

Any thoughts or interest in adding a bit of architecture here to extend the fact_cache part of the system so it can be exposed to more bins and other rule sets? My initial thought is to allow fast_path to define an array of other callbacks to check if the base page_cache_fastpath() comes up false...