Two ways to do this:
Use the original (non-beta) version of MakeCD. This can't burn mp3 so won't confuse your iso for one. The problem is you'll have to register it again with your code and then again when you copy the beta back when you're finished.
Or....
Rename LIBS:mpega.library to LIBS:mpega.library_, and reboot or flush. Now MakeCD will not be able to identify the iso as mpeg and will burn it normally. Just rename the lib back when you're finished.