FYI if you go with this suggestion, I'd call ahead to double check their policies on walk-ins. They recently closed the dog pavilion to the public in the interest of the dogs' wellbeing (less foot traffic = less stressed dogs), and I'm not sure if things are working differently for other animals too.

FYI for next time in case no one's mentioned it, most of the stops on the B branch are super close to stops on the C branch, which runs down Beacon. You can probably walk about 5-10 minutes depending on how fast you are from BU Central to St Mary's Street, which is the stop right before the C heads underground. If you're trying to get downtown that's likely easier and less stressful than trying to figure out buses or waiting for shuttles to get to you. (You can actually even walk to the D if you have a few additional minutes too. You'd be surprised how close all the green line branches are.)