They've been on Mac GUI Vault for a few years: Twiggy Mac prototype ROM
I also did some write-ups back in 2014, investigating the file format and ROM:
Twiggy Mac ROM disassembly
Report on Twiggy Mac disks
It isn't necessary to have the adapter. You can merge the contents into a Hi and Lo ROM as used on the production Macintosh board. Some people have already done it a few years ago with the newer Twiggy ROM version.
Thanks for doing this. I can't really understate how much I've gained from all of the materials the community has shared on outlets like Bitsavers, Macintosh Garden, the Internet Archive, and so on. I always try to pay it forward with stuff from my own collection.
Hey, this is a different take on a project I was considering taking on! My main focus was to output color to a modern external display rather than provide grayscale to the internal one, but this still sounds very interesting, and I definitely don't have any engineering experience in my project.
I was hesitating about including an FPGA in my project, because I feel like it should be possible to go without it, and leaving it off would definitely lower the price. But more power to you if you have the resources to handle that. Any chance your SoM has an exposed HDMI port we can try and get working post-launch?