I'm actually having a hard time following the "deprivation of magical power" argument, myself. If it were, say, a case of one magus stealing vis from another, or a magus embezzling vis from the covenant's stores, I can see that as being deprivation.
What I'm seeing here, though, is the council voting to use vis for a specific purpose (to wit, wages for Ysebrand). The charter specifically says that the members will abide by the decisions of the council. If, for example, the Council voted not to distribute the surplus one year (for whatever reason), would that also be a case for deprivation of magical power?
Keep in mind, also (and yes, this is metagaming) that there are still some vis sources out there that Siobhan rediscovered over the summer. She hasn't said anything to anyone yet (the Secretive flaw and all), but when she leaves come Christmas, she will leave a list of vis sources somewhere. If she finds out about them all (I haven't rolled yet), that could be another 26 pawns per year.
Halie can prosecute a case of Deprivation of Magical Power against whoever votes for the covenant to support Ysebrand if she wants. She may even be able to persuade people to vote in her favor at Tribunal in two years. But based on my understanding of the Code, it really doesn't look like a strong case.
The Pact of Crun Clach specifically says that "the resources of a covenant do not contribute to the magical power of a member of the Order of Hermes. Should I lose such resources to another magus, I have no recourse within the Peripheral Code of the Loch Leglean Tribunal."
It also says that "Items taken from a covenant as spoils of war henceforth belong to the taker. Magi may make reasonable requests for the return of certain individual items, and if complaint is found, I agree that the Loch Leglean Tribunal is the correct forum for redress."
So, like Normandy, in a way, but taken to a whole other level.
Right now, the covenant knows of 51 pawns of vis a year.