Since masochists like being in pain and tormented, locking them in prison should actually be up their alley. Add to the fact you're binding their hands and legs.
To make it not as OP, maybe the buff should come in effect after actually spending some time as a prisoner (for example: 24 hours) in order to avoid cheesing mood by quickly imprisoning and immediately releasing your pawns.
That is actually brilliant. Sometimes the simplest ideas are the best ones.