FWIW the outer diameter of the chamber of all the shotguns with the exception of the 10ga and 12ga Ultra Slug Hunter are the same. I have a 20ga USH that has the same OD as the standard 12ga on down to the 410, it is tapped, so why wouldn't it be OK for a standard 20ga barrel with the same dimensions? The standard 12ga is too thin, unless other measures are taken in to account.
Any professional gunsmith worth his salt should be able to safely drill and tap your 20ga.