I didn't know you could be at a 500ft level in Class A airspace (which
I've never flown in, except in a commercial carrier).

According to FAR =A7 91.135 : Operations in Class A airspace.
Except as provided in paragraph (d) of this section, each person
operating an aircraft in Class A airspace must conduct that operation
under instrument flight rules (IFR) and in compliance with the
following:

(a) Clearance. Operations may be conducted only under an ATC clearance
received prior to entering the airspace.

I've never received an en route clearance to an altitude other than in
the thousands, (except, of course during departure or approach).