Undead character should be able to enter the Hall of Echoes. Based on the lore you could design it either way, so I think gameplay has the deciding vote.
All player characters can communicate with the dead, at least the ones still around as ghosts. Undead may have extra abilities in that regard, depending on what is done for their racial traits.
There will need to be a disguise used in towns, so people don't flee or attack.
