hhmmmmm ...... I have made the supersize HSTs, but not had a problem.
Did all the same things you have done
EasyPeezy ... Guess I was lucky ... I'll just count my lucky stars!
hhmmmmm again ...... what about measuring it and marking it as normal ...
then
at the pinning stage, include a piece of paper (or tissue paper?) on the backside, the length of the seam lines?
Or maybe you wouldn't need to pin it, but lay it on the machine bed so that it gets sewn in place.
Then once stitched, carefully remove it without disrupting the stitching.
My thoughts are that the paper would help stabilize the diagonal/bias to prevent the stretch.
hhmmmmm #3 ...... what about a very thin iron-on stabilizer pressed in place the length of the seams?
Maybe a 2" piece that would straddle your two seams?