At most TS restaurants if you want to be assured of a seat near a certain time, make an ADR. If you choose to simply order dessert that's your choice.
As for fair to the server, well don't expect attentive service nor penalize the server because of that fact. Your lower ticket value is likely to yield a lower tip that say a similar sized group having dinner, including dessert would have. That is a generalization, you may tip heavy and they may tip light, but the server is going to play the averages.
As to fair to someone who wanted that seat for a full meal, again that is your call. You have a right to that seat, but if you would be disappointed to not get a shot at it because someone is choosing to have just dessert there, then make your decision with that in mind.
Bottom line: It is allowed, I'll leave it to you to add any of the disclaimers you've offered. It really isn't for another DISSER to determine your decision.