I know, but I would love to have it transparently, similar to some IOCCC guy that wrote a userland emulator to run Unix V4-v7 and up to BSD 2.1 binaries seamlessly. Similar to FreeBSD and NetBSD's compat(8) approach.

So you want CPU emul and a kernel personality. Kind of a tall ask, especially since you'll need much of Solaris anyway to run those vintage binaries.

See this: https://github.com/DoctorWkt/Apout

X11 calls can be just forwarded to the currently running server in your machine. The rest, yes, it must be implemented.

PD: I'm not saying IE for Unix would run under that; Solaris it's pretty much post BSD 2.1 and maybe from another Unix branch.

But code from NetBSD could be adapted for such task.