I'm mostly in a discussion with myself right now, but I still figured I'd post an update for the future googlers.

So I blew another axle yesterday, but I think I figured out what's happening. My chainline with the cassette cog was pretty close, but not 100%. Plus, I'm running a biopace ring, which makes chain tension a bit tricky: for the tension to not be too tight when I hit the tightest spot (around vertical cranks), it has to be quite a bit loose when it's the loosest (around horizontal). Not loose enough to jump off the chainring, but apparently loose enough to jump up on the next bigger cassette cog when I'm coasting with the cranks horizontal and hit a pothole or something. The next pedal step would inevitably bind the chain (bigger cog, so chain goes too tight), pull on the axle, and either bend it, pull the wheel out of the dropout, or both.

Fortunately yesterday's ride was to the LBS to pick up the spacer kit, so I just installed that, carefully set the chainline and tension, and now off to a test ride.