If you found a cheap universal that works the plasma's volume, that tends to imply that the plasma is using a regular TV's code. If this is the case, I would recommend getting a One For All or Radio Shack universal remote because most of them support a concept called "advanced codes", which you can use to program functions that are not automatically programmed as part of the built-in button layout.
Having said that, the MENU button most likely will be programmed, but you can still use advanced codes for other missing functions.
More info on my site
Rob
http://www.hifi-remote.com