A former Cambridge City Traffic Engineer told me I should treat this intersection as a two stage left. It's not at all marked, but there is enough space to safely do this, and it's worked well. Honestly, the left turn arrow green light is so short it's almost impossible for me to clear the intersection on my bike anyways, so I might as well just do a two stage left.

I'd be curious if the current Traffic Engineers recommend the same thing?

We will review the existing detection in order to determine if we can upgrade the loops to detect bikes. In the mean time, we would continue to advise that cyclists use the two-stage left turn maneuver via the east leg of the intersection. This would involve entering the intersection during the northbound through signal phase, stopping in front of the westbound through lanes, and waiting for the westbound through signal phase. This is a safer maneuver in that it does not require the cyclist to merge through three travel lanes.

If the official recommendation is a two stage left can some signage (or better yet, green boxes) be placed to indicate this?

We'll evaluate this location for two-stage left turn boxes in the Spring.