No, I think you're right -- "hot dog" started out as a colloquial name for a type of sausage (apparently as something of a joke, because dog meat was sometimes eaten in the area, and it was a low-quality product), and it is now the accepted name.

So it was an idiom, now it's canon.

Another good one might be "hot dish", which has an idiomatic meaning in the midwestern US, and is slowly spreading. Not sure if it's made it to the dictionary yet. (which dictionary becomes an important question -- I'd expect to see it in M-W before, say, OED)

I wonder where "sausage dog" fits into this lexicon