This is a great tip (or is that a workaround?!). However, you have to be careful- a dimension placed that way does not maintain its alignment to that edge, so a model change that causes the angle of the edge to change will make that dimension useless. No problem, if the change is big enough to notice; possibly a serious problem if it isn't.
Thank you all.
I discovered you can do the same thing as was in the video with a base line dimension, but you end up having to erase a few of the base line dimensions to leave the one you want. The key is where to hover your mouse before creating the dim.
Of course, I can do this in AutoCAD (surprisingly enough, also an Autodesk product) with a rotated dimension, and it doesn't require nearly as much hand-eye co-ordination. It's also documented in Acad.