In lieu of a trigger, have you considered scripting the restore process, including post-restore steps of creating the user you require? If it is a regular occurrence, maybe implement as a stored procedure, passing db name, path to data files, path to backup file.
MARCUS. Why dost thou laugh? It fits not with this hour.
TITUS. Why, I have not another tear to shed;
--Titus Andronicus, William Shakespeare