So I got a bit impatient after that last post and threw together a temporary test system to see how well MythTV+firewire input works these days, and how well XBMC works as a front end to it.
With cables trailing all over the apartment I temporarily hooked up our ancient DCT-6200 to my desktop, installed mythtv-backend and mariadb, and successfully set up my desktop as a temporary MythTV backend with only a couple of hiccups here and there. Connecting the XBMC system to it as a frontend was pretty easy, and XBMC certainly seems like a viable client experience with a bit of button behaviour tweaking and stuff.
Even using my Rawhide desktop as the backend with the storage on my NAS the performance wasn't bad - just a bit of jerkiness on sports channels - so I think it might be worthwhile throwing together a dedicated setup. I can pick up the newer DCX-3200 boxes capable of tuning h.264 channels pretty cheap on Craigslist, and I specced out a dedicated backend box for a couple hundred bucks, so it shouldn't break the bank.