A siege engine should be usable by anyone. No matter where it is. If no one is controlling it, an alb should be able to use an hibernian catapult, an hib should be able to use a mid trebuchet and so on.
I see problem with Tower oil because of wall climbers though. The realm being stolen from would need to be able to attack their own siege objects, although I guess if siege engines are set to being neutral to all they can be attacked by anyone, which in itself may become a problem when some assmad player starts killing 'friendly' siege engines during or before siege.
It's easy enough to ban for that sort of behavior, but another problem of similar sort is that a lot of AoE spells hitting players near a given siege engine are going to hit the siege engine too regardless of which realm is attacking/defending
But back to tower oils you'd have to always be weary of an assassin laying in wait at every tower you own to oil you More rarily if an assassin stows himself above the outer oil of a keep he has a shot at the entire zerg gathering at door for him to oil following an enemy's successful defense or having lost his own keep (actually this would be a risk to occur at every single keep take)
So I think oil should be omitted from the idea, but commandeering a paletone/treb inside an enemy keep would be fun stuff