I haven’t tried the specific load you are considering, but I have seen shot loads ( with shot cups) fired from rifled breechloading “shotguns” which were intended for shooting slugs. The rifling imparts a spin on the projectile, and centrifugal force then pulls the shot charge into a thin, widely dispersed pattern with a big hole right in its center. I witnessed this load fail to take down a falling plate at less than ten yards. That was in the hands of a very competent shooter.
I don’t think you’ll hurt your rifle by trying it, but the shot cup may leave a deposit of melted plastic in your bore which could be a pain to clean.
Again, I’m not speaking from experience, but it doesn’t sound like a good idea to me. A waste of time, money, shot, and powder, and you scatter plastic wads downrange.
Notchy Bob
EDIT: One other thing is that a true 28 gauge bore would be .550”. I don’t know if the wads/shot cups mentioned would load well in a .54 caliber rifle. Never tried it.