What is the source of the water and how cold does it get for how long?
I'm in WV, so we see some extended below freezing temperatures. Maybe not as cold as some areas, but we'll get close to 0*F every winter at some point. I've seen VT's installed outside on non-potable plant water systems which pull treated effluent from a manhole in the effluent piping. I've also seen vertical propeller pumps mounted outside pulling from aeration basins dischargeing into an open recycle channel. Both of these are in wastewater plants so the water stays above freezing temps.
When I was in NC, I saw some vertical inline pumps installed outside with no freezing protection, even though the temps would dip into the 20's at night - cold enough to bust the pipes in my garage with the heat off.
Look at your liquid temp, maybe provide a vacuum relif valve to allow the system to drain when not in use, and definately talk with your pump manufacturers to get their approval.