I have looked around at the available tools and I haven't seen anything that I am interested in. I think the ones that mimic actually calipers are far to bulky for my tastes - you have to move it around and rotate it. I would rather just click two points and get an estimate. That is why I thought I would ask about it here. I think it would be perfect for screenshot captor. I use it every day.
Presumably, if the user selected two points to define a line, you would then know how many pixels make up that line (roughly). The user would assign a length to that measurement. That would assign a scale factor at that zoom level. So any other lines drawn could have a measurement associated with it. I know this works because that is what I do by hand with printed engineering drawings. I'd be very happy with this functionality.
I would only want to zoom in to better define either end point of a line, making sure that it lined up as closely as possible with the feature that I want a measurement estimate from.