Any way to reverse engineer professional audio algorithms or those in e.g. rack units?
Not really. 

Anything that's a linear and time invariant is of course easy to characterize but most pro audio stuff doesn't fall into this category. It's typically dynamic, time variant, non-linear, signal dependent, etc. These types of algorithms are extremely difficult to quantify if you don't know the internal structure and the parameter space. 

Even if you get a full characterization, you may still not learn something about the way it's done. A time-invariant reverb, for example, can be measured through it's impulse response. However, you can't really tell how it's generated (unless it's a convolutional reverb in the first place).