I don't see why Mega Evolving should be restricted to A0. For most mons it's best to do it right away anyway but if you can find some reason to hold off for a specific action then why penalize that?
One problem is that mega-evolving a later action of the round can give the person ordering second. For example, if you are ordering first with a faster water-type against a Charizard that is holding a Charizardite Y in a 3 subs match, your orders might be something like this:
Rain Dance - Waterfall - Waterfall
If Charizard uses Sunny Day, use Waterfall that action and Rain Dance the next.
If Charizard uses Bide, use Rain Dance - Toxic - Waterfall.
If Charizard uses Counter when you would use Waterfall, use Hydro Pump.
However, that wouldn't leave you able to sub for something like this:
Thunder Punch - [MEGA-EVOLVE] Solar Beam - Solar Beam
Which would leave you wasting an action and ending with harsh sunlight for your two waterfalls.
This can also apply in other cases, like trying to use Skill Swap/Gastro Acid/Entrainment to disable a mega ability, like Mega Mawile's Huge Power, or using an attack that isn't very strong until the pokemon mega-evolves, like ground-type moves on a Charizard @ Charizardite X or using electric/ice moves on a Pinsir @ Pinsirite.