Thanks for the reply Nicki.
So, after tinkering a bit, it seems like what NGUI is trying to do is normalize the scale on the widget, and then set the appropriate font size. However, the font size isn't an option for the bitmap font, and so bitmap font labels that used scale are just totally screwed up.
It seems that changing my font to dynamic, and setting the "default font size" to the imported size of my bitmap font, and then attempting to load the 2.7 scene, results in something closer to what I expect. There are still quite a few widgets that are the wrong scale, apparently due to the upgrade process not handling certain cases where objects in the hierarchy were scaled (such as the scale on a root button object.)
If my understanding of this is correct, it'd be great if the 3.x upgrade notes could be updated with something to that effect. And maybe it shouldn't set the scale to <1,1,1> for labels that use bitmap fonts if that's going to result in a bad scale?