Nothing seems to stir up the grit like a slushy winter ride. I got back last Tuesday and my shoes and the bottoms of my pant legs were just coated in grit. Maybe because water can't freely flow off the road surface but only serves to carry the stuff onto you and your bike.

I think the bearing between the hub and sprocket carrier is already sealed with a felt washer. You could cut sealing rings out of felt for the cones which would block grit. Or, I guess you could just try it. No one seems to talk about how their s/a got trashed with grit.

anyway, find an old mountain frame which fits you and has horizontal dropouts. I think the mild steel frames hold up fine in the winter whereas some grades of aluminum are very susceptible to salt corrosion.