Well, today I got a wild hair and decided to buckle down on the output expander. I had done a board layout and was OK with the results, but I couldn't get up the nerve to run wires to, well, nothing. Doing some digging around in my bins of DIY stuff, I ran across a 2-wide MU blank with a bunch of holes drilled in it. That was all I needed!
After a little soldering and a little wiring, I now have a working (but crude) ArdCore output expander board. I'm going to put it through some torture testing, but I suspect we'll have these ready to ship before the end of the year. Yah!