Could a solution to the "would break compatibility with ongoing games" be solves by making a second iteration of the Gaia Project on the site, but the second one with the expansion available for it?
Ofter release the old version could then be discontintued.
The main downside would be a complete elo reset as people would need to work up the new version.
Might be too heavy a price, but I do strongly agree an Undo button would be very welcome.
I find the reason given in that bug forum thread that undos are against BGA policy a bit weird.
Literally every game I play here has undo possibilities up until points new information is revealed.