Try this one:
Even some of the old-tyme gun makers are using high-tech equipment!
Get a couple of those cheap handheld slimline laser pointers. You know which ones, the pocket pen-sized jobs or the small keyring jobs. Try to get the kind that uses AAA batteries. They are slimmer and cheaper to operate.
If you diassemble them, the guts are even smaller in diameter.
You need a couple to play with because sometimes the laser emitting head is misaligned (off boreline center).
Use a lathe to turn a housing (aluminum, brass, plastic, etc) to hold the laser pointer and be a slip fit into the BP muzzle loader bore.
To boresight, turn on the laser, and use a cleaning rod to seat into the bore at least half-way.
You boresight as normal, but at night or in a darkened hallway.
Once you've established boresight distance, it's a simple matter to determine scope elevation to put you "on target" at hunting distance. SPELL OUT TO THE CUSTOMER what that distance is, and write it down!
HTH
John