Aha! I have at last figured out how to get antibiotics into sick Ruby.
Mixing the gooey stuff with food stopped working after the first attempt. She'd take one sniff and walk away. So I had to return to the syringe.
However, trying to squeeze open her beak at the hinge—the way you'd squeeze open a cat's jaw--definitely didn't work. It was like trying to open a very small oyster. In my mammoth frustration I accidentally shot some onto her beak and she sideways-slurped it up. So I dribbled tiny bits along her beak until she'd finished it all.
Wrapping her up in a towel like a burrito also helped. (Excuse the unfortunate simile.) It's key to include the claws in the wrapping, which otherwise scrabble frantically and make things just that much more difficult.
About an hour after I hit upon this solution, I found it on Backyardchickens.com, deep in a post about something else. That chicken owner recommended the Beak Dribble for another reason: you won't accidentally get it down the bird's windpipe.
Ruby finally got her meds last night, and this morning, and again tonight! Phew.