Hi there!
Is it possible to have an EL panel where the letters are illuminated but everything else is transparent?
Maybe if you had one custom manufactured, but I don’t know if anyone has the technology available to do that. I’ve never seen what you’re after, I don’t know if anyone makes those.
You could fake it by [edge lighting acrylic panels that have been etched though](DIY Edge-Lit Acrylic Display - News - SparkFun Electronics)
I’d swear I’ve seen an exit sign with EL letters on a transparent sheet. Power supply was 120 vac direct and the letters were connected to each other with very fine wires that were invisible a few feet away.