Before I make my comment, I have to say while I have seen and handles TVM's work, I have never dealt with them.
From the little bit of info you posted, It's possible tyhat the extra time used in doing your special PB could be used more profitably in making 2 of their standard PBs.
Also, TVM is a SEMI-custom shop. They fill the void between an off-the-rack gun and a true custom builder (like Mike Brooks or Clay Smith, for example). From what you say, they aren't building you a rifle, they are modifying one they already make.
My suggestion is to go to the JJ Henry guys or find a true custom builder that specializes in 19th century guns.
Mario