I have a 12b Francotte single barrel trap gun which I plan on doing the same with and also think, as you do, that it would be a great way to hone some skills needed before tackling a double.
Thinking a 20bore chambered for Magtech brass cases would be fun or even stick with 12b as Casper did or maybe even cut and dress the barrel and try roundballs in it first. Never know, might be a great smoothbore right out of the gate.
Of course, a 500No.2 BPE is always an option. I seem to gravitate toward those.