Parking-space racks come out in the summer in the more bike-friendly cities near Boston, for several years now. So, yeah, it's definitely a thing here.

Really? I don't believe I've ever seen one in Boston/Cambridge/Somerville. What are the nearby bike friendly cities?

Srsly? Cambridge and Somerville usually have a few deployed around. I have actually used them. Union sq usually has one or two, but maybe not right now with all the construction.

And makes a lot of sense when demand is seasonal (e.g.: summer surges of bicyclists)

In the winter it would be hard to plow around them.