Vernier sights for the 1895CB are NOT a "dumb idea" at all!
The 26" rifle barrel itself has proven sufficently accurate for me to consider adding vernier sights. It is capable of reaching "way out there" for long range CAS events or casual plinking.. The tang is clean and has no serial number to cover up.
I've compared popular long-range sights for the Remington rolling block and Winchester 1885 design. The only foreseable problem is that you would have make up some sort of off-set sight base allow the long bolt to come back without hitting the sight. The sight base would have to mount behind your thumb in firing position.
Suitable tang sights would include various Lyman, Marbles, and winchester sight. Modern ones include many of the ones used for single-shot BPCR competition.
HTH
John