Substitute powders can be used as the main load if a booster charge of 5-10 grains of black powder is loaded first before the sub. powder is loaded.
According to the Hodgdon website:
Flintlock: To insure proper ignition in flintlock systems, 5 grains of FFFFG priming powder should be placed into the bore prior to loading the main charge of Triple Seven or Pyrodex. Consult the loading data in this brochure to determine the proper charge for the caliber firearm used and the chosen projectile. The main powder charge should be reduced by 5 grains to compensate for the addition of the priming powder. While holding the firearm vertically, slowly pour the measured charge of Triple Seven or Pyrodex into the barrel. Seat the projectile firmly against the powder. Make certain that there is no airspace between the powder and the projectile. *See WARNING below.
http://www.hodgdon.com/loading.html
There's at least one long rifle flintlock shooter who has posted range reports and chronograph results about the performance of his flintlocks when loaded with sub. powders with a booster charge. He's had absolutely reliable performance even with 777 using this method.
Of course he has a reliable high quality lock that virtually always fires for him every time without any swabbing between shots at all during his long shooting sessions.
There have also been a few reports about folks who have used only Pyrodex in the pan for priming. It can be made to work but not very reliably as there would also be many misfires too.
Every pound of black powder contains a greater or lesser percentage of "fines" in it - 4f black powder granules. Some of these can be sifted out and used for priming. But the remaining powder should be well mixed for best consistency and performance. Considering that many use 2f for the main charge anyway, any fines in the container shouldn't be missed if they were removed for priming.
And a lot of folks simply use 3F for both priming and their main load to save on needed to buy 2 different types of powder.
A pound of 4f may be enough to last the single flintlock rifle owner a lifetime.