You cannot control custom effect panels with scripts and that is that. The functions simply are not exposed and don't register as something that can be addressed.
if all you're doing is applying extractor to a pre defined channel and then trying get at it's output RGBA channels then you could just make presets of the channel you want using EXtractoR and then apply those presets with a script. from there you would sample the layer the way you would any footage item.
Sorry for this issue. Did you ever find a solution? Please let us know if you have or if you still need help.