I think a 20% downgrade could go along with a 50% increase in BFA. So if the reliever WHO NEVER STARTED has a max BFA of 10, boost it to 15 BFA while downgrading his ability 20%. When he hits 0 BFA let him deteriorate like any other exhausted starter, just a lot earlier in the game.

The idea should to discourage managers from starting pitchers who never started and to replicate as close as possible what would happen to a pitcher who never went beyond 2+ IP when asked to go longer.