Have a look at the WAR Fighter adjustable receiver at www.innovativearms.com
This receiver allows you to adjust the gas system easily and supposedly helps out a lot in suppressed applications. To the best of my knowledge you are correct in your assumption of one tax stamp as long as the suppressor is integral and cannot be removed from the rifle. Innovative arms may be able to build you a suppressed upper on that receiver.
Let us know what you find out. I am interested to see what you choose.