There is a lot of rifles on the market that meet your criteria , look into Ardesa , Lyman , Investarms and Pedersoli. They all have flintlock versions of their rifles.
BH we have been using slip fit bullets for as long as I have been shooting long range muzzleloaders , only difference is we use long lead bullets that are paper patched but I see no reason why it would not work with shorter bullets.
Sorry I read your post like my backside.....The Whitworth also uses a bullet that is sized by a die made of a piece off the barrel , so if it worked them why should it not work now.