I won't advocate over-stuffing a hotel room (because of KTTW issues for things like EMH, and because it IS a fire hazard), but y'all are being WAY too paranoid about being watched by resort staff; most of them have more important things to do with their working hours.
The only resort staff that is tasked with watching for over-capacity rooms are the housekeepers. They are supposed to report it if they see unauthorized beds in the room, or if the linen use is very excessive for the registered number of guests. I'm pretty sure that, in practice, if you keep the place tidy enough that you are not making extra work for them, and if you pick that mattress up and put it in closet before leaving for the day; they won't bother.
I know this because we DO often bring an extra bed, but not for an extra guest, and we have never once been questioned about it, perhaps because we keep a tidy room and recycle our bath linens. We have two opposite-gender children who don't do well sharing a bed, so the younger one gets a cot in rooms that only have two double beds. Not in a value, though; those are too small to do that without tripping over it. (FWIW, our preferred resort is S/D; they offer high-quality rollaway beds and enough room to comfortably accommodate them. S/D does not participate in the DP, but that's not an issue for us.)