hey @johnag
The trick here is to be supremely confident in what you ask for. Stand your ground and don’t waver. There will be people who will doubt you and think that they know how it should be done if they sense any second guessing yourself then you will loose the power to force this how you need it to be.
How I would do this is get everything shot in the US at 24fps (23.98). Then when it comes to the two formats get the editor to cut the US commercial @ 24fps (23.98) so a 30s=720fr
The AUS commercial just gets cut slightly longer. 30s=750fr
The difference between footage played at 24fps vs footage played at 25fps is so negligible that it isn’t worth doing any physical conversion. You just interpret the footage as 25ps or just cut a 30s commercial that lasts 750fr
Sound will need to make two mixes done specifically for your two different edits.
I usually run with the two edits. One 24fps (23.98) and one 25fps using the exact same footage.
If they ask for a 3:2 pull-down on your 23.98 footage ask then to check the delivery specs again 
but even so that is a very easy conversion.