In short - no, its not safe.
I had one a few years ago, also with the adjustable choke (its similar to a poly choke with some differences). I assume you mean the choke with a collet style constriction?
The problem with those on the 820B is that the shot column has chance to expand for an inch or so before being squeezed through the choked part. Seeing how steel does not not compress easily you are puting lots of pressure on that old adjustable choke. You might be OK with small shot sizes but not the larger sizes normally used for waterfowl. I would hate to blow the choke off the barrel in the field...
Option would be to have the choke cut off and use the cylinder bore for close up shots. Steel patterns tighter than lead so for close up shots with steel you are OK. I have used a cylinder bored old 870 (owner split the old full choke with steel!) for a few ducks at closer ranges (under 20m) and it worked fine.
Otherwise spring $279 for a new Mossberg...