I had designed something to that extent. One problem is it is a complete redesign on the system.
To go off what 1tonne said, as far as putting an HPA tank in the stock, while possible if you used a very small tank, it is highly unlikely. HPA tanks, NOT CO2, need to be recertified every X# of years. The US, EU, and many other countries, require it to not only protect shops, but owners as well. Due to the fact that HPA bottles can rupture and explode, rocket off, or a few other things.
We could do what TM did with the m870, but we would still have the same problem as the m870. The gas tanks apparently, in hot weather, expand causing the tank to stick inside the stock. I do not know the extent of the damage that it causes. Additionally if the stock were a tank running HPA, you would eventually need to replace the stock. If the rifle fails to be popular then you are going to be up the creek with out a paddle.
If it were designed to run off CO2 then it would be easier, still need sufficient expansion room so you do not freeze up the valve and reg, not that it would happen in a sniper rifle, but you would want to limit the number of points of possible failure. Which would mean hard-lining the connections through out the system, which would increase the end cost as those connections are expensive. Unless you ran it on a removable chassis that screws into place. Mind you that is what I would have done.
End of the day, it would be amazingly expensive. Sorry to be Mr. Buzzkillington there.