I'll start by acknowledging that I know this violates NEC. As I understand it, the reason that a 20 amp outlet can't be connected to a 15 amp circuit is that someone could potentially connect a 20 amp device, overload the circuit, and cause a fire. That makes sense.
Now, here's the question. Suppose all you have on hand is a 20 amp GFCI and you need to replace an outlet on a 15 amp circuit. (You will get a correct 15 amp outlet eventually, but you don't have it right now.) This circuit is on your own, private property - no one else will use it but you. You also don't own any 20 amp devices, so there is zero chance of one being connected to the circuit. (For sake of discussion, exclude situations like forgetting you did this, contractors using the circuit, sale of the property to an unaware party, etc. I acknowledge those are more good reasons for the NEC rule.)
Does connecting the 20 amp GFCI to the 15 amp circuit pose a danger just by its existence? That is, will it pose some risk just by being connected to the circuit, even if unused, or only used for 15 amp devices? Or, is it only potentially dangerous, because someone could connect something to it that they shouldn't?