I'm not sure if this is already known, but the Dark Brotherhood main headquarters are available in the original game data: NASYLUM. RMB (Night Mother Asylum). I couldn't test it in classic since this name does not follow standard RMB file naming convention, but the building does contain not only classic DB fellows, but the Night Mother in person as well (she uses texture 176.6).
So, let's hope this information will be useful for modders around there
To add to this, someone told me more about it. You were to be brought here to the nightmother for a quest to join the brotherhood. Obviously cut. Dialogue still exists in the preview to Daggerfall.
"Welcome to Nightside Asylum, sanctuary of the Dark"
Brotherhood. Here and elsewhere in Tamriel we can
train you and offer you opportunities to become a
"master assassin. We require only your loyalty,"
and your talents when needed. Will you join us?