That's actually a pretty good idea. I don't think it would be too hard to track requests fulfillment/refusal and possibly also track percentage of days you gave them an orgasm while they weren't away and allow the player release or relock with time added based on that.
Different rewards sounds good too, when the keyholder tells the player their time is up you could be given different choices of how you want to cum, with some (or all good ones) being disabled if you ignored the keyholder too much.
After the resulting sex scene you could then be asked if you want them to continue being the keyholder for another period, or if they were ignored too much the keyholder would decide the next forced period.
Did you use the temp chastity release action? Any feedback on that?