Chuck,
Bore scuffing or scrubbing by the steel shot is not the only potential problem with the "not approved for steel shot" barrels. The other part of it is the fact that the steel, when passing through the choke constriction, will not compress as will lead and softer non-toxics like bismuth and tungsten matrix. The larger the shot size and the tighter the constriction the greater the danger that the shot will not maneuver itself into position to pass through the choke. On rare occasions this does occur. I have seen numerous split barrels and choke tubes caused by shooters using Hevi-Shot through tight choke tubes for turkeys. Granted, Hevi-Shot is even less malleable than steel shot, but the danger is there just the same for a split muzzle.
I use steel occasionally in a BSS on ducks, but with a modified choke on the left I always feel I'm taking a bit of a chance. Fact of the matter is, for woodies in beaver ponds one could ream out all choke in both barrels, shoot steel and be safe from barrel damage. And have plenty of density in the pattern for the ducks.
Stan