I think a good UI would be to prompt it with something like "how far is that hole from the edge?" and it would measure it for you, and then "give me a slider to adjust it," and it gives you a slider that moves it in the appropriate direction. If there were already a dimension for that, it wouldn't help much, but sometimes the distance is derived.

I'd love to have that kind of UI for adjusting dimensions in regular (non-CAD) images. Or maybe adjusting the CSS on web pages?

I think that would make a lot of sense for non-CAD images, but the particular task you described there is do-able in just a few clicks in most CAD systems already. I think the AI would almost always take a longer time to do those kinds of actions than if you did it yourself.