Personally, I think it's OK to metagame some simple things to get around broken mechanics, and incomplete implementations, and has no effect on anyone else in the game at all. The only reason that a noble can't get out of an army without being a knight/lord is because the interface to do so has not yet been designed/coded. It's a workaround, not an exploit or abuse.
What's not OK to metagame is when a certain function is specifically not included on purpose. Or when you are using a function to do something other than the way it was designed, in order to achieve a different purpose.
In other words, I consider it OK to OOC work this out with a lord to get yourself out of the army. That's a workaround to an unintended situation that should/will be fixed.
But I don't consider it OK for the duke of the duchy that contains the realm's capital to swap allegiance to another realm, then immediately secede from that realm, simply because he's not allowed to secede from the original realm. That's an abuse of the game mechanics to do something that is specifically not allowed.
