I just found out about this page which mentions me and my old iBCS/NetBSD adventure to running Xenix binaries on NetBSD 4.x (Sadly 5.x broke xcoff). In there they also refer to this old redhat page on running a.out binaries on newish systems.
Apparently the ability to run old a.out stuff is still present as Alan Cox notes that “my 3.6rc kernel will still run a Rogue binary built in 1992. X is back compatible to apps far older than Linux.“
I’ll have to investigate later on.