A daily / hourly re-calculation of UA is therefore to be considered OK.

Indeed, that was one of my thoughts. In real-time, at current technical levels, it would be improbable that it could be done.

If the payout window stays 7 days, then I guess even every a 5 days recalc could be reasonable. With more users coming to Steem the amount of needed calculations would go up very fast. It would be a quite impressive piece of work I guess.

And if you never try, you'll never know.