I made once my own PCB Etching Bath with Air Bubbles Agitation, and I can tell you that although air bubbles do speed up the etching process, they are nowhere near to a direct agitation. With the term "direct agitation" I mean any means of agitation of the PCB or the liquid itself without an intermediate like the bubbles. For example, moving the PCB up and down in the liquid will speed up the etching process at least twice as fast as the bubbles.
Marco Franciosi made this etching bath with mechanical agitation. It has almost the same effect of moving the PCB into the liquid. For small productions i suppose it will be great and much faster than air bubbles. Go ahead and get ideas. Here is the link.