Smokeless in a cap and ball? Nah. Duplex loads were a thing in competitive BPCR and schuetzen type events in the past. Maybe it still is... Lyman has some loads listed in the old manuals for these. Generally it looks like 5 to 10 percent smokeless and the remainder being black was an accepted ratio. Not cap and ball as the original question asked, but smokeless has been used just in another form. I'd say these old guys had more knowledge on this stuff than the average shooter. And it likely started with the semi-smokeless stuff articap mentioned. Blending it for a bit more velocity and easier cleanup between shots is the reason I've seen mentioned for this experimentation.