The wxWindows system event implementation is incomplete and experimental, but is intended to be a platform-independent way of intercepting and sending events, including defining application-specific events and handlers.
Ultimately it is intended to be used as a way of testing wxWindows applications using scripts, although there are currently problems with this (especially with modal dialogs).
All this is documented more to provoke comments and suggestions, and jog my own memory, rather than to be used, since it has not been tested. However wxSendEvent will probably work if you instantiate the event structure properly for a command event type (see the code in wb_panel.cc for wxPanel::OnDefaultAction which uses wxSendEvent to send a command to the default button).
::wxRegisterExternalEventHandlers
::wxRemoveSecondaryEventHandler