The level_load instruction doesn't say anything about where the level has to be located. So maybe it would work like this
level_load(<z:\levelname.wmb> ); \\ the letter z is just a sample name for your CD-ROM
I hope it works, but a lot of loading functions in the engine expect that the file has to be in the gamefolder or in the save folder. If it doesn't work than I don't see a solution for your problem, only maybe a DLL workaround would help.
Michael