People gazed, open-mouthed.
Bill stood there, helpless.
I don't have a grammatical term for the structure, but it seems clear enough that open-mouthed and helpless are adjectives modifying people and Bill, not adverbs modifying gazed and stood.
Contrast: Bill stood there helplessly.
OK, I stuck a comma in there. But that's because I like commas.
Is there a name for this construction?
Am I imagining things ... ... again?
I've been checking all of my dictionaries, both US and UK, and Panjandrum, you may well be right. There's no indication that open-mouthed can be called an adverb. It appears to be classed strictly as an adjective.
But of course whatever the part of speech, there's nothing wrong with Godfrey's sentence. And I know of no hard-and-fast rule that an adjective may never follow a verb.
The advantage of having the adjective after the verb is that it becomes clearly descriptive rather than defining.
Open-mouthed people gazed (people who were open-mouthed gazed) - what did the closed-mouthed people do? Angry people waited(people who were angry waited) - what did the calm people do?
People gazed open-mouthed (people, who were open-mouthed, gazed) -clearly people were open-mouthed as they gazed. People waited angry (people, who were angry, waited) - clearly people were angry as they waited.