This has actually been done. The sub doesn't care what gas is behind it, all the sealed part does is act as an cushion for the woofer. A lighter less dense gas makes the sub behave as if it was in a larger airspace, at least vs the same volume of normal air. Fluids would have the opposite effect, so that would be useless. Also realize your cone isn't airproof and you'd have to refill it.
SPL competitors could, or maybe even have tried to cheat using gases. If you fill the cabin of your car with a gas that is more dense than normal air, you'll increase the cabin pressure leading to a higher SPL measurement. For a "sealed on the dash" type of run this could be an effective cheat. I know I've heard of people running their AC's to make the car colder, which also makes more dense air, but you could really take it to another level with a different gas lol.
There are a few AES papers with patents on this. They also use gases that are not only lighter than air, but gases that cool down when compressed, which is the opposite of most. This means that when the sub pushes inward, instead of having increasing pressure, the pressure actually remains fairly constant since the dropping temperature DECREASES the pressure and the sub moving inward increased it. That makes the box seem much larger than it is and there no change in compliance over stroke, sorta like an IB box, but you can make the overall Q anything you want, instead of just the woofers Q.. Kinda cool.
