Good point. I live in an area that never freezes so never experienced or thought about that problem. --- In areas where frost can be a problem, I would install two small diameter stainless steel bolts in the form of a cross at the vent top. Anything small enough to fit hopefully would not clog the vent and should be flush-able using a garden hose.
FWIW; Due to leaves and other debris we covered our vents & Chimmey cap with a wire screen-much like chicken coop wire. Then a very frigid winter came upon us and the condensation combined with the zero /subzero weather made for a build up of ice on chimney vent screen. Thankfully the heating system's (new boiler) safety mechanism turned the boiler off once it sensed the lack of venting or CO2.
That was the end of screening on any vents.