Catalyst Game Labs are now selling PDF copies of the Experimental Technical Readout: Steiner supplement for BattleTech.
From their website:
Experimental Technical Readout: Steiner samples the custom and prototype designs that have begun to turn up in testing labs and top-secret proving grounds of the war-torn Lyran Alliance. Each design featured in this book incorporates some of the latest technologies developed by the realms of the Inner Sphere as the war against the Word of Blake winds toward its final conclusion. These technologies—first described in Tactical Operations —have yet to achieve widespread production, but each has been prototyped and rushed to the field for their desperate consumers. Statistics and Record Sheets are included for 13 new experimental BattleMechs, vehicles, aerospace units, and battle armor designs presented in this book, ready for play in advanced BattleTech games.