How mad do luthiers get when signed artists play another brand?

[Sephiroth]

I see you're talking about Ibanez.

More like ESP/Schecter

[Rupe]

plus fender owns gretsch, so at gilmour is still in "the family"....

Not quite...Fender owns the distribution rights to Gretsch, not the actual company.

I recall back in the late 80's when an acquaintance of mine was endorsed by Ibanez. He said they would frequently send somebody out to their shows to make sure he was playing their guitars (they were based near Philly and so is Ibanez's US operations). He loved "out of the way" gigs where he could use other guitars...he hated playing the Ibbys (but liked their endorsement $$$).