I don't think I ever pondered this one! Perhaps try a couple test loads first, constructed as you mentioned but with a pinch of wood fire ash as a substitute? The stuff is inert so I don't see an issue, but I would cut the charge back a couple grains like you said.
Keep us posted. I might have to make a change in my will!
I did take part in a similar exercise at our trap club, with 12 gauge shotshells. Some ashes were added by sprinkling them in around the BB's before crimping, not unlike the Grex filler used in some non-toxic shot loads. We broke a few clays with him and then retired to the Trap Shack and hoisted some whiskey in his honor.
