Pecatonica will do swamped barrels on many of their patterns - or at least they did as of about a year or so ago, for an additional fee. The lock may be another story - you can have them leave the mortice non-inlet and leave as much wood as possible around the lock mortice. They are good about working with you on that stuff.
The Early Ketland will move your sear back a bit from where it would sit if you use a Siler - I don't know exactly how much but think it is at least 1/4 inch, maybe even 3/8. So you you would want to have them leave the triggers, guard, and sideplate uninlet as well if that is the way you want to go.
Keep in mind - when you start moving things around, the stock needs to be adjusted. If you move a lock back, the triggers move back, and thus the pull becomes shorter; the cheekpiece ends up moving a bit, etc. So the more wood you can have them leave for you to work with, the better.
On the other hand, if your stock is already set up with inlets, you can do some cosmetic things to a Siler to change the look a bit without changing the plate shape.
Good luck
Guy