If you have a compressed gas source powerful enough to cock a springer, you have only to channel it the other way and use it to shoot a pellet.
But this leads to something I was wondering about the other day. In a gun such as the one proposed, you have a volume of gas compressing the spring. If it were possible to release the gas down the barrel at the same time the sear is tripped, would the escaping gas be given a boost by the piston, resulting in a higher fps than spring or gas, alone? Remember, the gas will drop in pressure as it expands into the barrel, allowing the spring to recompress any in the compression chamber that has yet to bleed off.
Just what the world needs - a springer/PCP. Should keep both camps happy, especially if used with nitrous oxide.