Are the 22.04 image (64-bit & arm) build scripts accessible anywhere?
Very interesting question! I've done some "digging" and I eventually found the following video in YouTube posted by @Wimpy (Martin Wimpress) himself on 17th June 2022:
The description of that video contains the following text:
"This is the final test of Ubuntu MATE 22.04 on the Raspberry Pi I give an overview of the build script which I then release as open source for the first time (...)"
... and includes a link to the following GitHub repository, called "ubuntu-pi-image", authored by @Wimpy:
I hope this helps