Ivy and top D1 coaches are typically at FL and NXN to watch seniors they’ve already recruited and juniors they want to recruit.
Yes, it can happen occasionally that an Ivy coach has a slot remaining that late. It’s unusual but it can happen.
The situation you’re talking about though with a non-Ivy is a bit different as those coaches face fewer constraints and so haven’t typically “filled up” their recruiting classes at that point.
The logistics of going from a committed to D3 recruit to the Ivy likely letter carries some risk at that point, as presumably in the situation you’re describing there’d have been no pre-read and no OV.
I’m not sure it’s necessary to build an ethical rule for these sorts of edge cases. I tell recruits to act in good faith, communicate, and make decisions they’re comfortable with. If things change it’s probably better for everyone if the recruit chooses the path that works best for them.
I know of weird cases where recruits committed and had likely letters in hand but still went on official visits with the Ivy coach’s support. No way would I generalize from those unique circumstances to say other recruits should do that. (ETA: both found the grass wasn’t greener, which is what the Ivy coach expected).