Okay, so it turns out this was a bug after all.
The same trigger crashed my game from a couple weeks ago while I was streaming, and I couldn't track the cause then, either (very unusual stack trace and Windows error code). I'd put it away for a while to wait for more data, then just today Amphouse reported the same thing.
It's apparently been in there forever, but weapon overloading hasn't been used enough for it to show up much, and it doesn't happen all the time anyway, just under some strange circumstances I haven't been able to recreate. That said, I do believe I've figured out why it could theoretically occur, 99% sure here, and have refactored a bit of code to prevent it.
This goes down as one of the most convoluted bugs of the Alpha so far--took about 5 hours of sleuthing in all!
(So yeah, know that it's possible in Alpha 11 that firing an overloaded weapon could crash the game/UI due to a bug that should be fixed in Alpha 12.)