The windows plugin needs to be updated to be 1.1 compatible then you can do that.
Yep, I figured that out, sadly... I guess it is time to learn enough ruby to make that work on 1.1.
What was not clear from the doc is whether a local gem can be installed through the same mechanism as, but maybe that's just my English failing.
Ah. Well, maybe I should add an aside about that for people transitioning from 1.0.x. I'm really trying to avoid any mention of the word "gem" except in the development docs because I want users to stop thinking of Vagrant as a Ruby project.