Question: Keeping the strategic consequences of blowing up an actual satellite as technology demonstration aside, I want to assess it from a purely scientific/technological standpoint. 

To demonstrate the success of a developed A-SAT missile, what could be the alternative way(s) of testing, other than blowing up a real satellite?

For instance, I think a missile launched towards a parametric/specific target corridor in space with 'in-the-loop' simulated target tracking, could be used to see if systems performed well. It might have it's limitations, but I'm trying to see how close we can come to the test objectives achieved by blowing up a satellite, without actually blowing up one.

Is there a rationale behind choice of impact testing (boom!) with a physical target over other methods? If so, how significant is the advantage for the former?

Answer: 
One could in theory launch a very low orbit "satellite", about half-way around the world from where you plan to intercept it, intercept it at the peak point, and have it reenter in either case roughly at the point of origin. That starts to look a lot like a ballistic missile test, which is dangerous, and it really requires a launch point almost half-way around the globe. That requirement could bre relaxed, but at increased rocket performance. In addition, such objects won't look like satellites that have been deployed for some time, they probably can't deploy their solar panels that quickly, for instance.

Short of that, the next best thing is to intercept in a very low orbit, such as the Indian test did, or the US "Operation Burnt Frost", which technically wasn't an anti-satellite test, but an effort to make sure toxic fuel didn't crash and hurt or kill someone.
My original reaction was “hit recently-launched 200kg brick of dry ice”. Think of it as the sublime target: It’ll break up, increasing the surface area immensely, and shortly be gone. 

Unfortunately, the radar reflectivity of bulk CO2 is really poor. Even a 1m2 corner reflector would be very hard to track. 

Covering it or coating it pretty much defeats the purpose; even a couple microns of metal coating leave be a remnant hazard.