I think I might have found a system to make the pre-arrival upgrades work 100% of the time. Just made 3 new reservations using the below system and all three have been pre-upgraded
1) contacted the hotel directly by e-mail and asked a reservation related question
2) after their response informed them that I would go ahead with my reservation and would book over the LHW platform since I have Aurelian status and pre-arrival upgrades
3) got three times a response that they understood and they would take care of my pre-arrival upgrade if I would send them my booking confirmation from LHW after I made the booking, so I did
4) so that makes 3 successful bookings with confirmed pre-arrival, two within room category and one from deluxe room into suite level

At the same time, last weekend I stayed in a LHW hotel for 3 nights, booked a basic room, didn't receive a pre-arrival upgrade, so was pleasantly surprised when they upgraded me on arrival from the basic room to the 150m2 top suite, specifically saying that this was because of the Aurelian status.
It's these kind of experiences that make you appreciate a program.