AU Developers - Please PM Knightmare or MechRat if you need board or permission changes
That's not how copyrights work. The copyrights last at least 95 years. Since FASA was sold (or more accurately the BT property was sold) the copyrights on the old FASA books are now (theoretically) the property of Topps.
By public domain what do you mean exactly?I'd agree with Irose that the 'BattleTech' universe or setting is the property Topps currently and add that even in that there are certain gray areas which are disputed. The unseen is perhaps the best known example but there are other concepts which were appropriated in the games early days.