I’ve put my hand up a few times as well (albeit only on the forums) - of course it doesn’t help living in Australia.
I’ll take a guess and say the real reason is that you can’t flash the TCU in the S4’s ZF 6HP19 transmission externally like you can with the JHM / APR / etc flashing that you can do via the OBD2 port.
What does this mean? You need to drain it, remove the pan & filter that then exposes the mechatronic unit. That’s not the end of it, the whole mechatronic unit (looks like this) needs to come out in order to access the TCU. Once you’ve got the mechatronic removed you need to separate the TCU itself that is bolted directly to the unit. The TCU units are not sold separately to my knowledge - so if something happens to the board (here is what it looks like) you are liable for a full mechatronic unit from Audi ($US2000-3000 parts only).
Not meaning to sound like a buzz kill, I’ve just been reading up on these transmissions a lot lately and it makes sense that JHM would want to do it on site only. Of course I’m hoping that Jake is reading this and is now comfortable that I am able to get the TCU out and in to a static bag and shipped to Cali for the tune 
Whilst researching I’ve also found that there as a model split with both the ZF transmission hardware itself and with Audi’s factory TCU software. Apparently Audi updated the software due to shift shuddering and slipping some time around 06/07.
Anyhow, my two cents on the transmission - “D” is way too lax for my liking. It’s not necessarily the gear choice but the bite pressure on the clutch packs, it is just far too lazy on the shifts. I know Audi does this for comfort but come on…!