What are you using as a basis for calling this a hazardous location? Is there some building code that is calling this a hazrdous location? Just because there is natural gas piping involved doesn't make this an explosion hazard, if it did, any house with a natural gas water heater would have hazardous locations.
I'm certainly not faulting your concerns about safety with the above questions, but I just can't see why sealing is required.
Tom
