How much can you adjust the regulator? How high will it go? Is it removable if you wanted to replace with a Huma?
Can the .177 cal have the regulator pressure lowered so that the energy is closer to 18 ft-lbs, making it legal for field target (and to get more shots off the regulator)?
Yes, it can be done. Though, you really wouldn't need to. You can likely make all the changes needed via the hammer spring tension adjustment at the back of the action.
On a 3000psi fill, how many shots are regulated on the .177 cal?