It might not be necessary to go that far. For all I've heard, honor holds the same weight as gold here. Oaths sworn are powerful things - while we can't make them promise anything beyond the contract, a simple swearing to abide by it would bind everyone involved to each other in the right ways.

[no need to pull in magic unless fully necessary, when one could twist obligations instead.]

Mainly, I worry about a magical loophole built in by whoever designs it. Simplicity offers little wiggle room.