Is radar interferometry used, or feasible, for ground based astronomy? I've heard of "interferometric synthetic aperture radar" being used for Earth observation from satellites and airplanes. Is active radar used interferometrically for astronomical purposes too? Such as characterizing asteroids or space debris, or identifying volatiles in the polar craters of Mercury.

Unfortunately for this idea, Arecibo and FASTare located 173 longitudinal degrees apart and have limited fields of view, so they can't observe the same object simultaneously.
For obvious reasons you cannot use active radar to observe anything much further away than the Moon. There are plenty of phased-array microwave/radar passive sensing systems that map the energy in those wavelength bands that are emitted from stars, quasars and various other "hot" astronomical bodies.

To answer one of your examples, measurement of volatiles is usually done by measuring absorption spectra; which is tough to do with Mercury since we can't look at the solar-illuminated side (with special exceptions for certain short-lived orbital positions). Basically, that's why we send off probes to get close to various planets and asteroids, so that we can collect high-resolution visible and microwave images and, in some cases, do some active illumination (radar).  I am not aware of any interferometric active radar on such probes, but someone more knowledgeable may well correct me.