There is definitely spare capacity on BL past Maverick. While the 4xx routes do move a fair amount of people, a maxed frequency BL say 3 min, could easily split either after Maverick or Airport for a Chelsea stab.
How many times do we have to rediscuss this? If GJ is to provide useful transit, the only mode is LRVs, from West Station to Sullivan Sq. Time separated for FRA moves AT NIGHT, if you must, but trollies, people!
While the Springfield to Worcester section might have less demand at first, Worcester -Boston I would expect to be significant. I think that the NNEIRI study suggests otherwise. Are you familiar with it?