Obviously a big risk, if I were to do it I would set up an agreement making temporary wars like that allowed upon by both realms so its not just a hope they will have peace but the word of the realm.
Temporary wars, to arrest a priest with no real consequences? Sounds awfully close to a friendly conflict and hardly SMA to me. I see a real problem with needing to declare war to arrest priest, but setting up agreements for a war deceleration to basically mean nothing is not a great way to handle it.
From the Wiki on SMA
No powergaming. If the only reason you do something is game-mechanics, you should probably not do it. No declarations of war if you don't actually intend to fight, for example. No "assassinate me so the bounty stays within our realm", etc.
