Jay Honeck wrote:
I'm using Torx on the plane I'm building in several access panel
locations.

That is SO smart.

Is it illegal to subsitute Torx (or square-drive) screws in
certificated aircraft? If so, why?

I doubt it. And please note that I'm only using them in non-strucural areas
with one exception. The top skin between the panel (which is a bulkhead in
my plane) and the firewall. This will allow me total access behind the
panel. Also remember that the design here is for blind rivets. And the
designer gave his blessing to the modification. There are going to be about
60 of the little buggers in there.