I might add, I have NEVER heard or seen where a flight attendant got jumped up in priority. Pilots can, as I understand it, call crew schedule and get their category changed from D1 or D2 (normal non-rev code) to A1D which is "pos space must ride, bump a revenue passenger if you have to" code. HOWEVER, as MACH85ER said, that would be only if this was the only pilot available to work a flight which would cancel otherwise.
Except in very unusual circumstances all bases have more than enough flight attendants to cover a commuter who doesn't get on the plane. If there are no mechanicals/weather cancellations, the flight attendant gets a missed trip. A full a/c is not a valid reason for missing your sign-in.