Well I do not know much about Netplay plugins (only how to use them : )
I have already played with a Super Nintendo (Snes9x) which features netplay and always had problems with sync (woudn't exprect it to be anyway different on an n64 emu) but, if possible, here is an idea about a beginning, at least a little idea. would work only locally (one PC renders grafx and sound, the other only supplies a second controller (keyboard) and sends keystrokes over the lan)
writing an PJ64 (or compatible) server input plugin, which takes let's say controller 1 locally and 2 3 and 4 (if clients login) from network, else locally too or does then only feature one controller. the client would simply be a keylogger, wouldn't even need the rom or a fast grafx card. would be a way of getting you 133MHz pc for some n64 use - controller 2...
Tell me, would this be possible for writing? Don't know much about plugin specifications and network, but for a lan this would be possible and a first step to network play.
later on you can think of the client to feature grafx and sound (perhaps every minute or so the full n64 RAM from the server is sent to the client to ensure sync, would hang the game shortly but this is beta-stage *g*)
so far this idea. what do you think?