I wouldn’t use them. The extra grease will find its way To the brake pads. I repack mine every year with quality grease. I also replace the China bearings with American bearings. I lost a China bearings when I was 1,000 miles from home, it damaged the spindle I had to remove axle and get a new spindle installed.
These new axles are drilled and cross drilled to put the grease into the bearings not into the hub cavity. Unless you go crazy with the grease there is no problem with grease getting around the bearing seals and onto the brakes. Why do you re-pack your bearings every year, how often do you re-pack your truck's wheel bearings, never? We are not talking about a boat trailer that gets submerged in water. As far as a cover, most come with a rubber snap in cover just to keep the dust off and you can get them at most trailer suppliers.