I can't see as to how "lending" knights would be anything other than gaming the mechanics. The oath between the lord and knight is an oath of honor, not a simple contract for services rendered. It would seem to me that being "farmed out" and asked to swear a non-binding, or temporary oath to that lord, would be very insulting to the knight. Kind of like being treated like some common laborer or servant. The fact that you're offering to pay the knight a lump sum of gold on top of it, would seem to increase the "being treated like a common laborer" part of it.
Personally, I rather dislike the idea. If any of my characters were asked to do something like this, they would probably not only not go to the other region, but seek out a different lord who more values their noble service.
The few times I, and others, have reported such noble-swapping schemes on Dwilight, the answers have always come back as "handle it IC".
That's what estates have turned nobles to, passive labor. So while I agree with your sentiments, I rather blame the mechanics than the person adapting himself to them.
