I read an evaluation of Pinnacle by someone VERY knowledgeable in BP when the powder first came out, and the short answer is yes, maybe, if you really have to. Based on his experience, I'd say that if your firelock has an internally-coned touch-hole liner (so the powder is close to the flash) and a reasonably large touch-hole (at least 1/16"), then Pinnacle should ignite reasonably reliably, but likely with a slight hesitation compared to real gunpowder. If the touch-hole is of the traditional hole-drilled-into-barrel style, then you're likely to have more trouble. He did have reasonable success with grinding the powder down fine for priming, but it was still noticeably fussier and (IIRC) slower than real black.
If you can get even a limited amount of proper powder of any granulation, and your touch-hole is suitable for Pinnacle, then use the real stuff for priming. If you have a traditional touch-hole, then you might have to use duplex loads as one does with Pyrodex.
But your best bet is still to find the good stuff.
Good luck,
Joel