If you're experienced, then yes. But a good ULT or pneumag or even e-mag takes practice to walk the trigger faster than 8 or 10 bps. If you don't play that often and don't get to practice your finger twitching skills, then I think an AR frame can be faster and more reliable.
Maybe not.
This is the design I've been mulling over for a while now. (Forgive the simple drawing.) It relies on a rotating wheel that spins on a trigger pin inside the body of the frame. When the trigger is pulled, the wheel rotates clockwise and pushes the ball bearing against the sear arm, firing the marker. (I think a ball bearing would work best here, since it would spin against the sear and not rub and wear down.) Then when the trigger is released, the return spring pulls the wheel counter clockwise, pulling the ball bearing against the sear arm a second time and firing the marker again.
Obviously, proper placement of everything is really important. Also, I think the bottom left corner of the sear arm would need to be rounded off. Otherwise the ball bearing will jam. But I think the design has promise.
Also, like I said before, if I redesign the safety, I think I can make the frame switchable between semi and auto-response. If one safety setting stops the wheel right as it engages the sear, then when the return spring pulls back, it won't fire twice.