I don't know if it's been done or if there's any interest, but I'll be posting up the circuits for the Advantage and the Max controllers within about a week.

I haven't tested the circuit but it is the exact circuit from the NES Advantage. A and B connect to the appropriate pins on the 4021, I didn't include slow-mo because I never used it.

I also am going to do the Max which is a much smaller/easier circuit, however I always enjoyed being able to adjust the turbo speed on the Advantage. In some games like 1943 if you used full turbo it was like a "squirt" shot where as with the Advantage you turned turbo down until you had a steady stream of .50 cal destruction.