Can you serve as some sort of adviser/consultant for another chapter? You may not be able to "officially" be on National Staff because you are still an undergrad, but you might be able to stay involved with a nearby chapter.
Are none of your other chapter members interested in functioning as an alumnae chapter?
Sorry I'm not much help, but I'm not really sure how Phi Mu is set up in terms of staff, etc.
PsychTau