Best way to handle switching supply minimum load
As a block diagram (for one supply):

Current sensor -> comparator with hysteresis and reference-> MOSFET controlling resistor load.

You probably want a high-side current sensor. You'll also want to make sure that the relatively fast load switching does not cause undue transients in the supply rail under all conditions (high line/low line etc.). 



But generally speaking, you're probably better off specifying a supply that is designed to meet all your requirements out of the box.