In Canto 3 of Dante's Inferno, Dante's guide Virgil explains that some souls are not accepted by either Heaven or Hell. They didn't do any good in the world, but they didn't actively work against the forces of good, either. They just ignored the universal conflict between good and evil and drifted around aimlessly, pursuing their own empty interests and desires. Dante felt that much of humanity fit into this category. It seems like the Hollow Men are clearly meant to fall in a similar category. They are stuck on the bank of the River Acheron and cannot cross over into death, even though they are dead themselves.
The Hollow Men have no hope of ever crossing the River Acheron. They wait in vain without realizing it.
The Hollow Men have not been exiled from either Heaven or Hell except insofar as they have exiled themselves by forfeiting their humanity. They cannot achieve either salvation or damnation because they are not fully human.