Why, isn't that just called 'Super' key?
Super key could arguably apply to the shift keys, because you are using a super set of letters (or am I reaching too far)
But, the meta key already exists and on Windows it is the Windows key. We're going around in circles.
Ctrl - Ctrl - Ctrl
Super - Windows - Command
Left/Right Super - ?? - Open/Close Apple
Left Meta - Alt - Option
Right Meta - Alt Gr - Right Option?
Approaching an Apple keyboard for the first time, I naively thought Command would be Ctrl and was quite confused, since there also is Ctrl. But once you start a terminal it became quite clear that this was not the case. This is quite neat, it directly solves the SIGINT,SIGSTOP / Copy, Ctrl-D? confusion. Also having these operation as OS command means that all programs support them.
You're not though. The original letters are not available, so uppercase is not a superset of lowercase. (Unless you're just stating your opinion that the set of uppercase letters happens to be super)