So is "hemiteleia" an actual term of ancient Greek rhetoric, or did Howard coin it himself?

It seems to be applied in English solely to rhyming slang - and then only since 1983.

Nothing very specific is being elided in "New York's Finest." (Journalists have adapted the phrase to just about every other city police force in America, BTW.)

Think of "the old" and "the young," "the rich" and "the poor." Etc. The only name for this I can think of is "nominalization," which just means "noun-izing."