Doesn't seem like it's worth it. The LMA area is pretty self contained... it's not even a mile between where D and E is. People from the burbs taking the CR would be helped with better access from Ruggles or Lansdowne but I think most people would just continue to take D or E and just walk.
Part of the problem today is that the service is split between the D and the E. Service every 8 minutes on each of those branches doesn't equal an experienced headway of 4 minutes when going inbound. With "traditional" D-to-E, especially with an extension to Hyde Square or even Arborway, the majority of service would end up being on Huntington rather than Highland, so most people would just gravitate there.