Does anyone know of any farther objects where we've been able to detect the reflected wave?
1977, Arecibo: Galilean Satellites of Jupiter: 12.6-Centimeter Radar Observations Radar echoes.
1973, 12.6 cm: Radar Observations of the rings of Saturn Content is hidden behind Elsevier's paywall, so this might not be radar echoes.
NASA, 2006 implies the limit for radar echoes is at the moons of Saturn:
Looked for studies w radar echoes from moons of Uranus and Neptune, as well as Pluto/Charon, none found. The data may be out there.