its possible to bypass the dimension inventory group by throwing items into the portal, or by having an entity like a chest minecart go through the portal

I guess I could fix this for the vanilla minecarts and donkeys and whatnot and item entities in the vanilla portal... But if you have any other mods that add any inventories or teleport methods then it's a hopeless battle to try to prevent all the countless ways to smuggle out items. So all in all I don't really consider this a useful feature at least in the sense that it would actually be effective and secure. It's nice in principle but it's not really feasible in any kind of actual mod pack.