I haven't used a 28cm plastic Trixie wheel upside down, only the 20cm ones (which works well as long as the stand fits tightly - it doesn't always). But I have a plastic 28cm Trixie wheel and I can't see any reason why it wouldn't work upside down. It's one of the new ones, with a grey metal stand instead of black and in the new colours. I also think the stand might be sturdier than the old versions and it fits the wheel very tightly. I don't think it would fall off it was hung upside down. Possibly they loosen over time, or sometimes once you remove the stand the first time they never fit as well afterwards.
I've also hung a 28cm wooden Trixie wheel upside down by having threaded inserts into the base, then using threaded bolts and washers to hang it from the top of the cage.