The only way you could get a consistent and reliable pressure running through the system using just HPA, is if you had a direct remove attached to your pistol or mag at all times, using an tank as your reservoir.
However, this gives you two problems:
1) If you have the line running in to your magazine, good luck switching magazines when you run out of ammo
2) If you have the line running in to your pistol, you will need to redesign a completely new gas release system, as it is no longer running through the magazine.
The "easiest" method is direct remote in to the magazine, but then you run in to the problem of being stuck with just one magazine for the round.