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!