Y'all realize that "informal rules" are not actual laws.
This is something that can be 'changed in committee' or merely ignored with impunity should the leadership of the UN so desire.
Nowhere in the UN Charter itself can I find anything preventing Clinton from becoming Secy. Gen. should they so desire, and that is the only document that would approach having the force of law over this situation .
Besides; When have any inconvenient laws ever prevented the UN or the Clintons from doing what they want?
Never?