Doesn't matter if the lenses are impact proof. What the field operators are checking for is the ol' "is this a possible liability issue?"
If you mod a gas mask, the operators will see
- a modded mask, that has no history of protection
- the possibility of lenses popping
- a liability issue
Maybe you'll find an operator willing to take the risk, but at that point, I'd be raising an eyebrow about the sensibility of the operator. Remember, most of us have a history that often includes a RS firearm background, so we're often very safety & gun-safe oriented.
Personally, my advice is to abandon modding a gas mask and just use an accepted form of eye-pro. Keep the mask for cosplay and off the field.