Would it be possible to have copyright/signature remember the last used signature/position?
The way QU deals with print layout/feature settings that are often repeated by a user is with templates.
You could create a template for each size print and add a signature, or floating text, etc. It's up to you to decide whether this is a worthwhile exercise to create templates for your particular workflow
When the template has been made, save as a Layout and to use, select on the Print tab, Custom - LAYOUT predefined page set.
See first screen show below, note the "C" top left which shows there's a copyright/signature existing.
The second screen shot shows the page editor in HQ mode with an image added and the dice logo bottom right.
Re.
but I do not always want to apply this, particularly on large, to be matted, prints. Forgetting to turn off will cause wasted paper, ink, and time
It's pretty clear when a signature is in place from the "C" symbol on the preview & page editor. If you use the feature regularly, include it in your pre-print quality control check list.
Terry