I've never used a commercial drop-in but I have fitted my own barrels to several factory rifles. Full stocks or half stocks, it's easy enough and a lot cheaper than building another complete rifle and additional barrels don't take up nearly as much space in the closet as do additional guns.
I have one half stock with four different barrels and locks fitted to it, another with three barrels and locks and just finished fitting a second barrel and a side slapper lock to my old Blue Ridge flinter.
I can take just one rifle to rendezvous and compete in percussion, flint and smoothbore by just switching barrels and locks. If I'm hunting elk and decide I'd like to go after grouse this afternoon just a few minutes with a screw driver transforms my .54 percussion rifle into a 20 gauge flint fowler. Having additional barrels and lock along is nearly as good as having a spare complete rifle, just don't break the stock! :haha:
One can order a rifle from TVM with additional barrels fitted up and I suspect Matt would be willing to fit new barrels to existing TVM guns, one phone call would answer the question.