http://web.mit.edu/registrar/reg/xreg/ugradMITtoHarvard.html
EDIT: Just realized you already had this link, but this is the section where it is mentioned:
"Full-time sophomores, juniors, and seniors are eligible to cross-register for academic subjects offered at the Harvard Faculty of Arts and Sciences or one of Harvard’s professional schools. First-year students (including freshmen who declare Early Sophomore Standing) are not eligible for cross-registration.
Note: Undergraduates may not cross-register at the Harvard Business School, Harvard Extension School or Harvard Summer school."
So basically, Law, Medical, Government, we can take them all! =D
(I actually originally got this information off an MIT senior during CPW.)