(Sorry about my delayed response)
There is no "on walking" script hook, but you could give the armor a ridiculously high bonus to total energy and full energy recovery every turn to simulate movement's energy cost not mattering.
Your solution is very function.
The only draw back is that in my case I have a few items that use energy as part of their use cost so if I give an armor an incredibly high max energy the equipped soldier would be able to spam the item without consequence.
- type: STR_EXAMPLE_ITEM
costUse:
energy: 20
flatUse:
energy: true
This is hard part to add scripts as AI require to know how to handle it, if even I would allow to change cost it would be special flag on unit to give multiplier for cost, like "this unit spend 2 time more TU per tile than other units".
It wouldn't bother me if it was only able to be used exclusively by player controlled units.
Either by a hard restriction that the AI (Alien/Civ) units can't wear armor with this variable or just by them ignoring it when present on an armor and losing energy as they normally do when walking.
So long as X-Com Soldier's can spend 0 energy when walking while wearing this specific armor, I would be perfectly happy with it.