One of the speakers I'm interested in is the Graham Audio LS8/1 ($10k), designed by Derek Hughes, the son of the engineer who designed some famous BBC speakers. The LS8/1 got a very positive review in TAS (if anyone's interested, the review has a lot of fascinating info in it: https://www.theabsolutesound.com/articles/graham-ls8-1-loudspeaker/). So I emailed Graham Audio and got the following reply directly from Derek:
There is no reason why the speaker could not be used upside down, it will not harm it. Interestingly, the BBC often used to used their BC1 speakers inverted when they were used near field, so yes that should be fine.