Hi Adam -
I think I win the "worst response time ever" badge. Sheesh. Sorry for not getting back with you - I (unknowingly) did not have email notifications turned on, and well, this got buried
The Rhubarb is in early production. Retail price is ~$350. The link is not done yet, but the spec sheet is available here
In your follow-on post, you have the right idea. The RPi becomes the supervisory controller, the uC (Arduino, STM32, etc) becomes the realtime controller. This is what the Beaglebone series of SBCs do.
Also, the board looks great How did it work out?