As I said, it's not always a reliable option, the next person you ask may say, "sure, send it in", then Ilion will say no way!!
As I said, they have fitted them as accessory barrels, but only as an option when the ordered barrel was out of stock after they received the order. As Dinny suggested, the Tracker II is rifled with sights, you could order the 20ga and have it drilled and tapped, dunno if they would, Gardner did, but a gunsmith certainly would, maybe you'd be lucky and they would be out of the TII and sub the ULSH!!!
Tim
http://www.hr1871.com/Support/accessoryProgram.asp