Is it possible to use logic statement in the base file editor of Endpoint manager?
ie, If this extension does not have a voicemail box enabled, then SUBSCRIBE for MWI = 0 (no) else SUBSCRIBE for MWI = 1 (yes)
you can submit a feature request at http://issues.freepbx.org … the only downside to this idea is that you would have to rebuild the config(s) and reboot just to enable voicemail