I am trying to implement a (trivial) virtual list control demo.

What example are you trying to follow?

What does wxListCtrl::OnGetItemText documentation say?

This function must be overridden in the derived class for a control with wxLC_VIRTUAL style.

Your MyForm does not derive a Wx::ListCtrl, it not isa Wx::ListCtrl like a MyListCtrl isa Wx::ListCtrl or Wx::DemoModules::wxListCtrl::Virtual isa Wx::ListCtrl

inheritance is the keyword, when to derive a class you must inherit from it