I had been thinking of something similar for larger clusters, since they get expensive to light with a separate ematch or other starter for every motor. Ducting the flash seems to be the way to go, based on the unreliability I've often seen reported for open pans. I was thinking of using motor mounts to slip over the end of the motors rather than ducting straight into the nozzles though.I used a flash pan to light the 30 motors.
You can see it in the very bottom right of the picture. I used a section of coupler that I had laying around. Put a bulkhead in the bottom and made 2 centering ring templates from my open rocket sim. Drilled a 3/8 inch hole for each motor and inserted a 3/8 inch diameter piece of metal tubing in each hole with about 3/4 of an inch of metal tubing sticking out the top. To use it I put about 8 grams of pyrodex inside the pan. Slide the rocket down the rail and match the tubing up to each motor. The weight of the rocket keeps the flash pan in place. Light the pyrodex with an ematch, it blows flame right into the motor nozzles and off she goes.
I cant take credit for the design, found it on an old post somewhere on the forum. But it sure does work.
Are you using Pyrodex for slower burning than BP, or for another reason?