Probably could expect some sort of CDOT equipment to be used New Haven-Springfield.
I doubt that MBTA equipment will get used -- there isn't much practical application for it in New England. Since the T doesn't have any electrics, all they can contribute is single level coaches without motive power -- and there are probably more than enough of those available elsewhere that come with their own motive power (NJT Arrows, MARC).
I also don't think that Amtrak woul ever put commuter equipment on a full length NEC run (i.e., Boston to DC), as that would be a bit abusive to passengers.
Assuming that Amtrak's biggest challenge in the NE is electric locomotives, not coaches, maybe they could swap MARC 6 or 8 P40's for their electrics for a week. I'm not sure if the MARC AEM7's & HHP8's have all the necesary ACSES gear for traveling to New England, but they could certainly free up Amtrak's own units for those runs. Plus, NJ and Philly and NYC could use a splash of color.