Neat plugin idea.
Is the idea behind the meta 1) On Trigger and 2) Off Trigger that they fire whenever there is a transition from ALL devices in the group are OFF to at least ONE of them is ON (for #1), and when there is a transition of ANY of the devices being ON to ALL of them are OFF (#2)? If so, then the actions could be to update a KeypadLinc LED button to represent the ON state of a group of devices.
Can you instead of creating a custom plugin Device instead just create a custom plugin Trigger? It seems like that is more of what you want in this case.