Going perhaps even further in this direction, the 3 hour window could shift by an hour every day. All the other solutions I see to the time zone problem are pushing the idea in the opposite direction, back a little toward normalcy, and have their own problems anyway (eg power users who just run multiple accounts and are the ones doing a lot of posting).

A shifting window would be even more "slow internet". Of course it would be a different vibe; you'd have stuff to catch up on the days it's in prime time for you. As-is it seems like it'd have more of a real-time vibe.