First the question:
For every milestone, there is a 1xp situation that can be achieved virtually infinite times in a scene. That's ok. BUT:
The Heart of a Prince
1 XP: when you first use your powers to give a support asset to an ally.
from The Thing's datafile. What does it mean "you first use"? It means you can get the 1xp only one time per scene?
Then the house rule:
You can spend 1PP to have your hero break in the scene if she was not already here. It's up to you to explain how and why she managed to arrive (or maybe she was always there!). Think of the primetime adventures' rule wich does the same thing by spending fanmail.
My intent is to follow the same logic of how the watcher can rearrange the heroes' current positions to force them to use a particular affiliation by spending doom pool.