Some of the things I've seen from other posts include:
1. Are you SURE the grease is fully packed around the bearings? Sometimes it's just surface-packed and not really filled completely in the entire cavity.
2. Are the new bearings the right size?
3. Could it still be the
brakes? Try driving with them adjusted wide open and disconnected from the controller so they can't possibly activate and rub. We had a 1978 13'
Burro and didn't really need the
brakes, so you should be safe driving for a while without them.
You are correct that the hub shouldn't be that hot from driving. Good luck with the investigation!