There is no real legal mechanism for revocation.

There is. Parliament has the right to control how citizenship is obtained and lost, it's governed by federal law. If Alberta actually passes a referendum and gets it past the clarity act, you can expect that Canada will be looking into updating the Citizenship Act. They can create whatever legal mechanism they want as long as they don't leave people stateless or apply it in a way that is unjust (according to the supreme court and constitution).

It would be a huge legal battle but it's something that the federal government will hold over the province in a secession negotiation, and the final settlement between Canada and Alberta would include an agreement about how to handle citizenships.