Quick update for future browsers of this thread.

The married segment availability is quite specific. It isn't possible to generally link to a TATL flight - only specific combinations are allowed, otherwise the availability on the domestic leg drops back to standalone availability.

If I was booking LAS-IAH-LON straight through, it would be available down to T class. If LAS-IAH is split in anyway from the specific IAH-LON flight, it goes back up to U class.

My plan is thwarted...