So part of the problem you are gonna face is bolt length and weight. With pure blowback, you depend on the mass of the bolt to help hold things closed. With a 9mm AR bolt, if you shorten it you start loosing weight from the heaviest spot on it. You need some way to replace the weight or compensate for it being gone (ie something else besides bolt mass to keep chamber from unlocking too soon).
The next part of the problem is the spring, either used to completely stop and control that bolt from slamming into the back end at speed (at least slow it down a good bit) but also to bring the bolt back forward with enough energy to easily strip a round off the mag and chamber it and close up tight.
For the second part, running a rod where the gas tube would normally be and having it attached to the bolt/carrier where the gas key would be, with a heavy spring on the other side that is compressed as the chamber opens might work (think AK or Galil piston system style). Or a spring on each side with just a rod in the middle, something like a T handle on it push a spring on each side.
All FALs use two springs - one up front to return the piston, and one in back to slow and return the bolt. The para model still has the front spring set up, but the rear spring is changed from the rat tail pushing on the spring in the stock to a pair of much smaller springs that run in the upper above the bolt carrier.