I'm sure that some folks have done it more than a few times, but there's always a risk that it won't work as intended.
I wouldn't trust doing it unless the bore was already beat up and had pitting or rust.
And I wouldn't shoot them in your new antique shotgun unless there was a zombie invasion.
I even cringe when I see folks firing steel airgun BB's from any kind of smooth bore and those are probably much softer steel than bearing balls.
If there was a way to securely encase some steel balls inside of a plastic shot cup without breaking open, then perhaps it would become a mini solid slug or a type of bean bag round.
But I wouldn't know how to guarantee that it wouldn't break open inside of the bore and releasing the balls.
And then only by firing one at a paper target would you be able to tell if it held together or not.
It's not without some risk and IMO not a good idea to try with any antique gun.
I'm only trying to explore the topic since it's not worth spending the money to buy plastic shot wads to try this when the money could be used to buy lead shot instead.
Maybe you can glue some of those steel balls into the hollow point of bullets so they will expand better, or to create a more aerodynamic round.
Or just wait for a rust bucket ML to come along, then you can play around with them.