OK..wanted to share this.I may have found a workflow to do what I'm talking about...
Since Scribus fields can execute math, you simply scale up the size of the image box, then apply the same multiplier to the image size.
In plain language, if I crop the image the way I like it, but want to enlarge or shrink that *crop,*, I multiply the size of the box, say by 2 (link the aspect ratio) then do the same for the image (aspect ratio also linked).
The only problem with this method is that the image moves relative to the box, but all you have to do is scootch it over.
The image doesn't always change position inside the image box after doing this procedure..I'll have to experiment more to see if there's a pattern.
Of course, mind the "center of change" when resizing the box..if you want the top left corner to stay put, click that radio button.
Not ideal, but way less fiddling around than what I'd been doing before, and the sizing gets nailed.