This might not be as niche as you might think. I'm one of the
Linux kernel maintainers for s390. Many of us do the vast majority of
their development work natively on s390 systems via SSH from Fedora
laptops.
I first wanted to echo and confirm what Niklas says here.
The crux of this issue seems to be "the code in the X server that does this is
virtually untested" so would more attention being paid to this code help? I can't
make any promises, but it would be valuable to know if this, or something else, is needed.
I will also bring this to the attention of the Open Mainframe Project Linux Distributions
Working Group, since all of the distros use this byte-swapped code.