I had a little bit of time over the weekend to buffet up the SIO routines. While they still aren't giving 100% error-free transmission in all cases, they're a lot better than my first cut. I used the 6502 code that this guy wrote (http://www.horus.com/~hias/atari/) as a guide to the 6809 version, which has proven pretty stable at 38,400bps.
So with that in place, I can now boot into DriveWire and access disk services.
The next thing to tackle is a more stable booting environment. Currently my boot ROM is just a hacked together lump of code, but I'm planning on bringing Brett Gordon's Forth-based CoCoBoot into the picture soon.
Boards should be here mid-week, and other parts are trickling in day by day from UPS and the post office. Hopefully this weekend I'll have some time to put together a board.