Been there, done that (in 1996), using the NetBSD 68k SunOS compatibility layer on my A3000UX.
I had to tweak the kernel code, because the compatibility layer would either run Sun 2 binaries or Sun 3 binaries, but not both. The Sun 2 was not binary compatible with the Sun 3, but it worked the other way round. The NetBSD code did not see it quite like that
Anyway, cross compilation was not a solution to the problem. Porting was the goal.
I share your opinion, but as I had to learn, this is not yet possible. Both Cloanto and Hyperion only have licenses to use certain material, but the licenser alone can contribute historic code to the Computer History Museum.
I, for one, would like to see Exec and Intuition to be in the museum's collection.