The problem with using substitute powders in flintlocks is that the flashpoint for the subs is so much higher than it is for Black Powder, it is difficult to ignite. Now, the Pyrodex pellets are suppose to have some amount of black powder on one end of the pellet. If you load a starter load of BLACK POWDER down the barrel before loading the pellets, they can be ignited, altho reports indicate that both ignition and SDV are irratic.
I don't think anything you can do with any breechplug is going to have much to do with whether they pellets work or not. You have to get enough heat inside the barrel from the burning priming powder to ignite the main charge. Getting heat there is not a problem. Getting enough heat to ignite the higher flash point powders IS.