Ok, so I just found out that for the H77 Pro4-M port [1], S3 resume is listed as working.
I built a ROM for that target and flashed it to my B75M-ITX, and now resume from S3 just works. (Other things are broken though, like ethernet.)
So I guess I have to compare these two targets now and find out what would be the fix?
[1] https://review.coreboot.org/c/coreboot/+/45317