Fun stuff. I'd be tempted to either a) run a deployment step to freeze CSS locally or b) do some nginx reverse proxy magic to grab the latest CSS and cache it for a long time (I.e. only grabbing it from GAE once per revision).

That's a fairly consequential thingee to have on a server with availability independent of one's own.

If folks want I'll OSS code for this, although at a dozen lines in Rails it is barely worth typing git clone...