> Throw in the Oxford comma and George has become a policeman: “Through the window she saw George, a policeman, and several onlookers”.

The problem here might also be that of artificially limiting the solution to commas. We have alternative or better punctuation available to demarcate the interrupting thought. One could emphasize the relationship better either with an em dash or with parentheses:

“Through the window she saw George — a policeman — and several onlookers.”

“Through the window she saw George (a policeman) and several onlookers.”

In text either of those could better express the different emphasis which verbally would be conveyed by a shift in the speaker's tone.