We were able to resolve this with support: There was a company in SY01500 that was not in SQL ... Once we ran the Clear Companies script from KB 871680 we were able to configure Data Mart in RU 5.
(I was being thrown by the db_datareader reference, thinking it was referring to the creation of the DDM database. Now that I know the answer, I realize it was trying to read each GP company database and not finding the user had read access to the non-existent database.)
I hope this helps others!