Don't see why steel shot will do any harm, OK it's not proofed for superior steel but anything else will be fine if it is in good condition. I took a 1899 Watson Bros in recently for reproof and passed fine for any standard nitro load, only thing is that 2 3/4" chambers are the way to go for the future as 2 1/2" loads might be harder to get in steel. If the barrels are up to it have the chambers lengthened, get it reproofed and enjoy for years to come.
Got a rather nice W&S 700 if anyone is interested...