At the company I was at (stoppped operating on June 30 this year) but an ALPA carrier, since the average life expectancy of an FO was about 4-5 months before they quit for greener pastures, we could never keep any FO's around -
So the company solution was to offer upgrade to everyone once they reached 1500tt - in the theory that once you made captain you were less likely to leave - whether there was a need for captains or not. (It did kind of work in the beginning.) At one point we had 160 captains to 75-80 FOs.
If you were senior enough, you got to fly as captain - if you weren't senior enough - you got reserve and maybe got to fly as captain when you covered for some captain's sick or vacation, but more than likely got to assigned to fly an unfilled FO's line at normal captain's pay. - which most people didn't care for - they wanted the turbine PIC time.
Lineholder CA's could be junior assigned for 200% to fly as FO, but not an FO's line for the month.
There was no contractual language preventing assignment of captains to FO lines and originally there was no requirement for right seat qualifications at all - until one captain who was hired straight into the captain seat and had never flown in the right seat complained that he couldn't fly as FO because he'd never received training to fly in the right seat and had no clue as to what the FOs did on their side of the cockpit. Then all captains had to do a right seat qual.