You asked, so this is my two cents...
With all due respect to the OP and owner, I can't stand those ridiculous signed guitars; authentic or not. You see them everywhere and they're always the same. Some worthless junk guitar that in no way has any connection to the artist in question, in a tacky frame, adorned with signatures that somehow ascribes it more meaning or value beyond what it is.
But maybe I just don't get the collecting of signatures in general. Either way, those framed guitars are a tacky embarrassment.
On a different note, I once almost bought a Rickenbacker that was ex Marty Wilson-Piper of The Church from a guitar shop in Sydney. It had been in a Hard Rock Cafe somewhere and had his signature on the body. It was a great rare guitar regardless of its pedigree, but the signature was my biggest impediment from buying it.
And while I'm at it, during another trip to Sydney I went into a prominent music shop in Bondi and was blown away to find not one, but two Vox AC30s ('90s 30th anniversary models with greenbacks) complete with yellow "Midnight Oil - Sydney" Sprint hand stencils on the sides. On picking myself up off the floor and asking the shop assistant about them he told me they were Martin's, they were his live rig and he had traded them in on some new ones ahead of going out on the road again.
I so wish I'd bought at least one.
Adrian