The Bucks boys used a really simple release. The pin was hidden under the toeplate. The (straight) rod pushed against a (leaf) spring driven into the wood. The spring released a catch that was peened into the patchbox lid.
Muzzleloader Builders Supply sells the kit that has everything you need (#10160).
I have pictures of an original gun with this feature as well as the setup I used for one of my guns. I don't have time right now to post them but, if anyone's interested, send me your e-mail address and I'll forward them.