Lots of unnatural behaviors come out in captivity and it probably wouldn't be too hard to do some crosses. If you can mix python species, kings and corns, etc., I don't think garters are special and uncrossable. Perhaps a few specific combinations wouldn't work but I'd be very surprised if it didn't work at all. Subspecies of the same species are very likely to be compatible.

Before getting into snakes I did a lot of work with Betta splendens. In the last 10-15 years the asian farmers have crossed a lot of different species from the Betta genus in with B. splendens. B. imbellis was one of the first, which yielded the copper color and the metallic layer. It's hard to buy B.splendens today and be confident that it is genetically pure. I really like the copper color but I refuse to buy obviously crossbred fish.

I hope garters don't end up the same way, but to think it can't happen would be a bit naive.