RSVP literally means "Please respond", however it seems to have turned itself into a noun in common usage:
"What's your RSVP for the party?"
Is it acceptable to refer to a person's attendance status as their RSVP ?
The conventional meaning of the abbreviation is (from New Oxford American Dictionary):
Modern usage of the term has pushed the boundaries of its use, and the Corpus of Contemporary American English includes examples of it being used as pretty much everything form a noun, a verb, an adjective, etc.
It appears as such in fiction, magazines and printed news, so you can consider it's pretty common (though not particularly formal).
It's not their attendance status per se, it's their response. It's acceptable, if uncommon, colloquial usage. I'm more used to seeing RSVP used informally as a verb: