It wouldn't be possible. The street grid doesn't align with the rail grid. Only the Green Line's B, C, and E branches are busable, plus Red between Charles and Porter.
You can't have a night route paralleling Orange south of the city. You have to use either Washington or Huntington. Using Huntington is most ideal to serve Brookline Village, not Forest Hills. A Nubian route is best suited to pick up Forest Hills. North of the city, a route into Malden must serve Everett, not Wellington. With Red, you have to use Dot Ave, so serving JFK on a night bus is out of the question no matter what. If you're not on a main arterial that's directly aimed at downtown, then it is likely there is no good way for night service to realistically reach you. A night bus has to make the most direct trajectory between downtown and the terminal as fast as possible to complete a cycle.
One thing with Boston is that areas needing night service is not a single hub, so if you're in Longwood or Airport and want to get to Forest Hills or Chelsea, the night network potentially may require a circuitous route. In addition, not all routes map cleanly to a single running time takt. The city is also just a
smidge too big to get hourly service. You'd get routes that need 80 minutes to cycle back to Downtown. Put 2 buses onto the route and you're at 40 minute headways.