well I gave a quick look and searched for custom flint lock rifle builders and I came up with dozens and dozens of builders
maybe your not searching things as well as you can
but you are right most builder, get into what they like and tend to stick mostly with THERE type or ideal set up
but about any good builder can MAKE you what you want, if you know the exact details in the build you want!
when you find builders online, its a game of sending them emails, on your basic;'s and getting a feel what there interest level is, and from there making phone calls and talking to them
its not as hard as many think it is
but it can be very time consuming, but when you want a custom gun, your entering a game of making the TIME to get what you want right, failure ion your part will NOT be the builders fault
and trust me I know of many people that had custom guns build that were not happy, and it boiled down to they didn;t be specific enough when having the gun built, they left a LOT of options and build parts up to the smith building hings, and then AFTER they get it, they say, I should have done this or that!
and one of the very BEST ways to NOT have that happen is spending TIME learning what YOU want, expressing things with the builder to make SURE your both on the same page
costs can get carried away when doing this, as the more specific you are, it normally ends up costing more
but in the end, you have what you want which in itself has values many guys don;t see up front, but pays off when its done and you have exactly what you want over CLOSE to it!
I am NOT a big flint loc person
there are a few guys on this site that are way deeper involved in with them and I would gather a few might add better specif info on builders for you, better than what I can help you with!
but if I was you, I'd be searching the web better, as I seem to have found WAY more builders it seems than you did in one search
from there again, its spending emails, making phone calls and talking to each and all you find till you find the one you want to do your work/build!
if not that, Maybe try and find a more dedicated Muzzleloader/flintlock forum, have to think there are some out there over a more general hunting forum like this(not bashing this suite either just saying a more specific site might have decades more help for you)
as there are some very intense highly invested and involved muzzleloaders out there and if you look at prices on some of them custom build one's you will see it reflects that there is a strong following in them to support there builds!
best of luck to you, and please post back what you end up with and share some pic's of your build when done, more than me here I am sure would like to see things!