ok, i kind of took a crack at this on my blog, so here’s the interesting part:
if you get the dependents of a master, and check all the controllers you will see the following on the 'driving' channel.
#(Controller:Position_Rotation_Scale, ReferenceTarget:Reaction_Master, Controller:Position_Reaction, ReferenceTarget:Reaction_Set, ReferenceTarget:Reaction_Manager, ReferenceTarget:ReferenceTarget, ReferenceTarget:Scene, Controller:Position_Rotation_Scale, $Box:box02 @ [58.426544,76.195091,0.000000], $Box:box01 @ [-42.007244,70.495964,0.000000], ReferenceTarget:NodeSelection, ReferenceTarget:ReferenceTarget, ReferenceTarget:ReferenceTarget)
See the 'ReferenceTarget:Reaction_Master' bit? This is the distinguishing feature of a Master, and it's located on the channel that drives the slaves. You can query it to see what node it is driving, then go to that node and check it's controllers to see what is driven:
exprForMAXObject (refs.dependents $.position.controller)
"<<Reaction Master instance>>"
getclassname (refs.dependents $.position.controller)
Very hacky, but at least it’s possible.