I haven't actually read this thread much (sorry) so I am unsure of any of the realistic suggestions...
But has putting small breeder fish (such as guppies) in a thought? Suprisingly enough if it's smaller than a gold fishes mouth it's gold fish food. As guppies are live breeders they can be a good go.
They'll eat mosquito larvae to.
Over winter most fish slow down considerably and in fact outdoor ones may stop eating full stop. Algae stops growing, and even bacteria slow down or stop (especially in cold to very cold climates).
Things all come back to life again in spring. So if there's breeder fish in there they'll come back to life and the larger fish will have a continually supply.
Even in the tropics fish slow to almost stop over winter. My barra would survive on one feed a month. Then as soon as the weather warmed up they were scoffing food back left right and centre.
Go to the local creek and use a bait catcher to catch a handful of local fish - of course these are the ones that will most likely survive the conditions in your area.
Hope this wasn't off topic or already answered
