As has been stated, "Special Steel" has to do with the material the barrel is made from, not steel shot. Per Browning, "Do not shoot steel shot through any Belgian made gun". Of course, this could be a CYA thing from Browning, but they are pretty emphatic about steel in Belgian guns. CIP, on the other hand, has a series of recommendations for the use of steel shot in older guns not proofed for steel. Basically, don't use any load over 1 1/8oz, no shot larger than #4, no MV higher than 1400fps and no choke tighter than modified. And, yes, the CIP recommendation was published well after Roster invented the thick petaled wad.
That said, there are a ton of old A5s out there that people are shooting steel through with little of no problems. About the biggest problem that one can experience is a bulge in the barrel right behind the choke. And while this walnut is noticeable, and probably an annoyance to anyone who has one, it is purely cosmetic and will not effect the way the gun shoots. About the safest thing to do is to see if you can find a steel safe barrel for the gun, especially if it is of any sentimental value.
I feel slightly sorry for a man who has never patterned his gun, who has no idea how far his chosen load will retain killing penetration. But I'm extremely sorry for the ducks he shoots at beyond the killing range of his gun and load - Bob Brister