hey matt, I don't know if this helps but Northeastern has a lot that is free to park in on the weekends and it is right next to the ruggles station, which is 5 stops away from Downtown Crossing(right near Orpheum).
If that helps, the parking lot is on Columbus.
coming from 93 you take exit 20 and turn right onto Mass Ave. Then 6 or so lights up you turn left on Columbus(street after Treemont) and it is all the way down on the right.