We currently have 15 permanent residents serving in the Canadian Armed Forces, and 140 other nationalities serving on exchange with Canada. Of those exchange officers or soldiers, about 30 would have at least three years' service with the Canadian Armed Forces. Those would be the people who qualify. It's a very small group.
Traditionally, as you say, we've had recourse to it when a permanent resident has a competency, a form of expertise that other Canadians don't have in the Canadian Armed Forces. It recognizes the fact that all those who have the right to work here in principle have the right to work in the private sector or the public sector.
It's not a widespread practice in the Canadian Forces, but it's certainly not prohibited by law.