no scrums or lineouts I presume? Surely the union team when defending in their own half would just pilfer the ball every ruck.
Wests Magpies beat Randwick in 2015 under the same rules. Union rules in the attacking half does favour the union side, but it's very continuous which suits the league conditioning, and the league side will make breaks from their own half and score some long range tries. Even when tackled they can sometimes go through 2 or 3 quick phases under union rules if they've got some momentum and the defence is going backwards. If they get slowed down they'll get turned over pretty easily, but there's a 60 second shot clock anyway, so you have to go fairly helter skelter in attack.
There's a potential hack for the union team to just camp themselves on the halfway line in defence - concede that first 50m every time to the league side but it would mean your defence is set when they have to attack under union rules. They'd presumably have to ban that tactic as I can't see how the league team would have a chance against it.
It's all pretty gimmicky and I can't imagine either code wants to risk losing to the other at the professional level. What seems more possible would be an Australia vs New Zealand hybrid match, with both teams a 50/50 mix of union and league players.