The “Capture Screenshot to the clipboard” toolbutton saves to a file if Ctrl is pressed instead of copying to the clipboard, so there is precedent.
I really don’t like this idea. It hides the zoom mode, and makes Python tracing of a zoom that much harder.
I’d vote to just add new tool buttons. However you implement the new mode, you may want to do it for both the whole scene bounding box and have another button to do it for the active source.