Flintlock? Argh! Well, Pete Allen does make a 1 1/8 in breechplug that can be used for flint. I did one once in .66 caliber and it seemed to work out OK. Mike Nesbitt wrote up a little review of it in Muzzleloader, I think, years ago.
I am not aware of any English style breechplugs in 1 1/4" which would be better for .69 or .72, IMHO.
He also makes a 1 1/8" percussion breechplug that I was really thinking about mounting an 8 bore barrel on for a little big bore stalking rifle. I thought that would be really neat, but alas, just another unfinished project that went by the wayside. It would have probably finished out at about 10-11 pounds, maybe run 150 or so grain charges and have a ball.