If no one else answers throughout the course of the day, I'll look in my Marconi signal-book when I get home tonight. I remember it being it there because the original owner of the book pencil-lined through all the ships sunk during WWI, including Lusi.
The call letters allotted to Lusitania were MFA. MSU was unassigned to any station, ship or land-based, at least in 1913/4 (the years covered by my Marconi signal-book). Just in case anyone wonders, MGY was reassigned to the SS Filey after Titanic.