rar or zip work well, but you´d lock yourself out from using OS9 to rejoin the parts in this case.
and if you use alladdins partial zip in stuff it deluxe you might end up with something which requires stuffit engine.
if you only want to split a DVD image onto 5 CDs without the need to look inside,
split & concat would be the easiest solution, but this is also OSX only.
an ideal solution (i.e. readable on 3 operating systems) obviously does not exist.
though i am not sure if not maybe disc copy does something like that, too?