Re: Regarding the ULTRIX and OSF1 compats

[[ building "Ultrix"-ABI binaries on non-Ultrix platforms, to test COMPAT_ULTRIX ]]

I  think x86 or arm makes a lot more sense than Sparc.  All Ultrix platforms were little-endian.
I don't recall ever making COMPAT_ULTRIX endian-neutral. Or whether any of the struct-copying code cares.

You'd also have to create fake system headers with the Ultrix/4.3BSD-sized structs, and compile your userland test app using those headers.
And all the ioctl codes, using the COMPAT struct sies.    Not particularly hard, but tedious and error-prone.