Oops! Didn't go far enough. I was actually thinking of making up a tool to push down into the door past the glass and fish for the lock mechanism to open it, but after rereading Matt's description - several times - I don't think that would work.
I worked for years as an automotive trim mechanic (rattles, squeaks, water leaks, wind whistles, etc.) and on a few occasions had to open doors which had become permanently locked. Most of the time the above-mentioned trick worked, but if, as Matt says, the door handles and lock button appear to be working as designed, the only thing left is the actual rotary door latch. They were used by a lot of auto makers starting in the mid/late 1930's. I would see if a good working example can be found to check out how the rotary wheel is released to open the door. Having said that there may be only one option left, starting with removing the front seat (been there, done that) to gain access to as many of the door panel screws/clips as possible. Pulling the door panel out far enough to be able to get a long thin blade screwdriver into the latch to release it, assuming it can be seen to do that, is a possibility. There will likely be some damage to the door panel which sucks on a nice original car but.... Good luck, however you do it, Matt.