I would say it is an extremely bad idea to go to a high end restaurant in costume. It does not follow the guidelines and it is not appropriate for a restaurant where other guests are making an effort to create an upscale atmosphere.
I also think you would feel extremely out of place. There are guests there spending $1000+ on the evening, and will be dressed up.
Just my opinion, but if I were working the restaurant I would decline anyone in costume entry, and I hope Disney would.
Hey, who are you to judge people on this forum?
OH...riiiiight...uh...never mind, then.
Seriously, for the OP, I'd think a little less about how out of place YOU'D feel and maybe about how the other patrons would feel. For the money people drop to dine there, in part because of the scenery and ambiance, it could be considered rude to show up in a way designed to draw attention to you.
Also, depending on the size and awkwardness of the costume, it might be awkward seating you and navigating around you.
And I'll say this - were I dining in California Grill and you showed up in a costume that scared my kids, I'd be pretty vocal about you either moving out of our sightline or changing into civilian clothes.
Either way, for crying out loud, they HAVE PUBLIC BATHROOMS IN THE HOTEL. Wear something a little nicer, then when you're done eating, duck in and change. That would be the most considerate thing to do.
I know others have aid they've seen people dining in costume before, and I'm sure the staff at CG do their best to accommodate. At least the people dressed as pirates weren't acting like pirates.
And here's some food for thought (expensive, since it's Cali Grill) - Darth Vader is now a Disney property. WDW will routinely ask people to remove costumes if a: they look too much like the real thing and b: they cover the face. A Darth Vader costume so scary that it could scare children on the bus (yet somehow miraculously not scare children in a restaurant) would probably look pretty authentic AND obscure your face. So it seems like dressing like a Swarf would be your better bet, and it'd hav to be so drastically different from an actual officlal costume, it probably won't be more than a hat and a pull-off beard and comical clothes. So pick a thinner costume, put nice pants and a nice shirt over it, look respectable for dinner, take it off and stow it in a bag after dinner, rent a locker to throw it in for the night, and everyone's happy.