I think everyone has raised some excellent points and have greatly enjoyed this posting.
My only experience was with a CVA "Squirrel" rifle - .32 caliber - I believe it had a 24" inch barrel but I sold it a few yars aqo so perhaps someone can correct me on that if I'm wrong. It was a nice shooting little rifle. Up until the time I bought it used, I had always shot .36 or .40 caliber rifles that I custom built and they had 36 or 42 inch barrels. I loved the little .32 though and it was a sweet shooting rifle.
Without getting in to twists, etc. - I think it all depends on what you want to do with the rifle. If your intent is to shoot shorter ranges - 25 & 50 yards - I think you'd have a lot of fun with it. Shooting at longer ranges are going to make you "learn" your rifle and adjust for drop and windage.
With my eyesight problems, I went to an underhammer .36 and love it. I used to shoot with a guy who had a .32 underhammer with about a 30 inch barrel and he could "clean our clocks" most of the time at 25 and 50 yard targets - this was competing against all kinds of stock and custom built rifles.
I think your idea is a great one and I'd encourage you to try it. If you are using an underhammer action, you can always make an interchangeable barrel in a greater length and / or caliber. You could have the .32 barrel and if you wanted to try longer ranges, have a longer barrel either in 32, 36 or 40 caliber. Your ramrod pipes, etc. will be on the barrel along with the ramrod so it presents no problem when interchanging barrels. The combinations you could have of caliber / barrel lenght are limited only by your pocketbook.
Thanks for the great post and I hope you'll keep us posted on what you do!