Using a less clunky way to connect a ReWire instrument wouldn't prevent you from doing clever things to the MIDI stream being passed to it. Reason, for example, is an instrument with a built-in sequencer. So is UltraBeat. Which one is easier to set up on an Instrument track? Which one works better once you get it there?
Yes, it would be possible to create an AU instrument plugin which is an AU client to its host, and a ReWire host to a ReWire client. This would be similar to how ProTools does ReWire. The question is, could it work around the fact that Logic already snatches a ReWire connection to Reason once you start Reason? This I can't answer without getting the ReWire development stuff from Propellerhead, and doing some fiddling.
Frankly, I'm not convinced about the Environment, but as I've only been using Logic for a month, I'll give it time to convince me.