Applying custom MasterPage using Extendscript

Is it even possible?

The closest I've gotten to it is about 2 steps shy of actually applying the custom masterpage.

Fcodes([app.GetNamedCommand("MasterPageUsage").Fcode])

This gets the dialog box open but how do you choose the custom masterpage by name and apply it without opening a dialog box?

And on the flip side, if a custom masterpage is already set, how do you change it back to the default, "Right"?

Cheers