I've asked Elydeon and this is what she said:
"My first thought is that people need to be reminded that a regional champion is far outside the paygrade of most trainers, and thus a trainer who is capable of defeating one should also be way outside the paygrade of anyone in this thread. But then, this is Alola, it only barely has a regional champion - so I guess that works.
The second thought is that basically, so long as the Mega Houndoom is treated as an NPC (that is, use of the particular character and the actions taken by them is not limited to this one person) and it is treated as a particularly-difficult boss encounter event for this entire group, that should be acceptable. Mega Evolutions are very powerful, but not unbeatable by regular pokemon. It would just be one hell of a fight."
Basically, it's a yes.