Do you already have the .69 roundballs? I did play with some .58s at one time. They loaded in a shortened WAA12, with just the gas seal portion of a second WAA12 underneath and a folded crimp. Accuracy was relatively "good" on large targets through a modified or tighter choke. I was just playing with them to use up an auction lot extra, but my 12 GA "musket" was pretty fun in the backyard with a modest charge of Red Dot.
I would look to BPI for components specific to a .690 RB if you are looking for anything approaching high performance.