I've never played in any of those realms, so I personally haven't been aware of any such rules. Nor have do I remember ever hearing about any problems with this in the past.
In Sanguis Astroism on Dwilight, we had a rule that if you wanted to hold certain elder ranks, you had to be a priest. If anyone wanted to hold those ranks, they were expected to convert to the priesthood. I don't remember anyone ever complaining that this violated the IRs. I don't remember anyone ever challenging that rule, though. No one ever refused to be a priest after they were appointed, or complained that excluding non-priests violated their right to choose their own class.
I kind of agree with vonGenf. If you handle it IC, I don't really see a problem. You're allowed to say things like: "I don't think you should be our ruler. You're a priest, and we don't want to let any religion have that much power over our realm." or "Infiltrators are dishonorable, and we shouldn't allow them in our realm. They should all be banned." You're not limiting their choice of character class, nor telling them what class to be. A character is responsible for the choices they make, including any possible negative effects of those choices. They can choose to be a priest or infiltrator, but they have to accept that there are ways they may be limited by that choice.
If it were up to me to make the call on this, I'd have to say that as long as it is handled IC, it sounds legitimate. Some variety and flavor is a welcome change.
