All DS hardware is disabled entirely when GBA mode is enabled. While yes, you can set a border before launching a GBA game, once in GBA mode there is no way to change that border or access the additional pixels in any way. When playing a GBA game there is no way to access the wifi hardware at all.
If you could boot a GBA game straight from the slot and have it add its own border, then yes it would be a good indication of the DS hardware being accessible from within GBA mode, but unfortunately this is not the case. It's more of a case of storing some data in memory before shutting out access to it.