Chris,
I have investigated having this done by Teague a couple of times with English guns that needed some more choke.
Both times they said the barrel walls were too thin in the muzzle area to be safe to do.
I suspect that this is the norm rather than the exception unless we are taking about a recently rebarrelled or sleeved gun.
As others have said, I think you are going to have to jug choke (providing there is enough wall thickness), play with cartridges or buy another of my sleeved guns! wink
Having said that, if one had got the wall thickness to do it, I would fit a set of Teague thin-wall interchangeable chokes: very inconspicuous and ultimate adaptability.
Best,
Toby