By analogy with French, from which we picked up half our vocabulary and a number of grammatical structures, I'd say "come" is a past participle acting as a preposition by virtue of its being usable as an adjective. I find the example awkward, but not incorrect.
"Come winter, we'll be feeling very cold in Canada." = Once winter has come...
This is to be differentiated from "Come what may," where it seems the word is an imperative, daring fate to deliver whatever it wants.