John Ross wrote:
As I understand it, in order to fly (legally) above 17,500 feet, the
pilot must be instrument-rated and file an IFR flight plan.

In sunny, cloudless conditions can you file an IFR flight plan and go
over 17,500 if your plane (like my Pitts 12) is not equipped to fly at
night or in bad weather, i.e. no lights or gyros?

IIRC, flight above FL180 also requires dual transponders.