Not a bad idea

I like it.
Of course it would be a bit more complex, and require some serious effort to work out the bugs in the system for the system installing it...but that's going to happen with any detection system.
basically it sounds a lot like an advanced version of Semi-active system used by older air to air missiles, like the Sparrow. It would work, it wouldn't be as precise as a conventional sesnors, but it would act like a wide area motion tracker.
By comparing snapshots of the reflected energy, a system could spot moving objects by detecting changes in the reflected energy....Basically a computer would look at snap shot A, and snapshot B and run an algorithm to spot differences in the two.
Adding filters to kick out known objects, and scanning for friendly transponders, and roughly estimate the size and velocity of objects in the area would allow the system to send less garbage data to the human operators.
While it wouldn't be very useful for accurate tracking and ranging, it would give human operators a rough idea of size, shape, velocity, and course. More advanced systems and patrol craft could then sweep with their own sensors to get a better idea of what they were dealing with
In combat it would also make a nasty intruder glow like a light bulb to any friendly sensor system, especially the smaller, cheaper, sensors of a missile/torpedo. it would allow small fast less easily detected( read less likely to be recognized as a manufactured object) vessels fire off missiles before the enemy realized the rock they were cruising near is actually a disguised combat vessel or platform.
of course I wouldn't want to be the poor soul tasked with looking at all the returns spit out by the system. Especially in a system with a lot of rouge bodies zipping in and out.......Hmmmmm well there's another use for it. Detecting rouge bodies as they enter the system.