I vaguely remember reading that there's simh support for it, but I don't
see it.
I don't think there's any specific simh support for a Multinet port, but
you can of course run Multinet "thru" simh (i.e. install Multinet on a simh
VAX/VMS system and tunnel DECnet thru the simh emulated Ethernet
controller).
I assume you're asking about a simh invention that talks to a remote
Multinet VAX/VMS system on the simh host side and pretends to be some other
device (like a DMR/DUP/DMV/whatever) on the simulated side - I've never seen
anything like that.
DECnet/Linux can supposedly emulate a Multinet tunnel. Is that what
you're thinking of? But in that case you're running DECnet on the Linux
host and simh has nothing to do with it.
Bob