Thing is, if this is even stated anywhere at all (and I do not see it on the Rules and Policies Wiki page), it seems like more of a policy. The Magistrates are for adjudicating violations of the the IRs and the Social Contract. As Tom said, I don't believe that this falls into either category unless you want to argue that it's either a bug exploit or a Fair Play issue. IMO either one would be a stretch.
If Tom feels that this kind of thing should be officially proscribed, it is up to him to make or clarify the rule and determine who would enforce it. Otherwise, I think this is something that needs to be handled by the community, IG.
That's a nice sentiment, but based on what feyeleanor has been saying, people have been trying to do exactly that for years without success.
The problem with "you have to handle that sort of thing IG" is that the type of people who are willing to bend the rules like this tend to be the type of people who are also willing to min/max and, well, bend the rules in other ways, just to get ahead.
And when a realm that's already more powerful than the others around it takes a measure like this to "protect" itself from foreign religions, you have a recipe for a middle finger to the notion of RP and in-game common sense that just gets to sit there and show how useless those ideas are when they are left to the players to enforce for everything short of a blatant violation of the IR or Social Contract.
Although personally, I think it falls squarely under the "fair play" clause of the Social Contract, like many similar "well, nothing says I
can't do it, so screw you" issues.
