Thanks for your comprehensive response!
BTW there is a new Yśn which just came out. I think this will wind up being the best approach to IoT stuff. In other words the two-tiered approach with a WiFi chip running OWRT and an AVR running the actual control with a bridge between them just like the Yun has.
I guess what I would love to see stripped down, lightweight Perl at the OpenWRT level for HTML-based UI development and MQTT and stuff like that. Some simple, fast, lightweight Perl combo that has some Web-UI development stuff and MTTQ stack too, plus some common crypto stuff.
Also your approach using rbpi and an avr via serial is great too. But I think that the two tiered approach using owrt and the avr on the same board is awesome.