FINALLY got my engine and transmission separated!!
After about 40 attempts with welding a nut onto the broken bolt, I gave up and decided on a different route. I drilled straight down into the bolt, and used one of the others as a template to know how far down to go so that I wouldn't accidentally drill into the threads in the engine block. It took me about 2 hours of drilling - the woes of Harbor Freight drill bits and a cordless drill - but I finally got it drilled all the way down. Then it was as simple as wiggling the engine and transmission with some pry bars, and I could see the bolt start to recede and a gap opening up. A minute or so of rocking it back and forth it and it finally let go!
Aftermath video here:
https://youtu.be/TfuGHvy6TK0