Uavionix has their Tailbeaconx ($2499 us) which is a mode S transponder with diversity built in. Requires a control head as well. Can use their AV-20-E ($895) or 30-E ($1595) or will interface with MGL and some GRT EFIS at this point. If bundled they take $200 off the 20 or $300 off the 30...