You can force the valves closed by disconnecting them I think it is, but as you said it isn't advised by
Capristo due to back pressure at high rpm. You could install some of those silencer disks perhaps on the end of the exhaust tips to try to help muffle the sound a bit, and of course during sound testing you could take it easy by the microphone and hopefully skate by.
You might also consider tracks that don't have such noise restrictions in place if there are some near you.
A curved L-pipe on teh end of the exhaust to point it away from the microphone might also allow you to pass.