IF source is off canvas, then I'm not sure about having to process it anyway (wasted effort)
Are you aware of the option (not sure on other OS, for Windows, it is using the ALT-key on the middle edges) to crop a source?
In my case, I would take a 4K video, and do a combination of crop and resize to get desired framing on the canvas. By cropping, the source edges are visible to me, no funky work-arounds required
I am not immediately imagining a scenario where one would want to do what you are asking (not saying it doesn't exist, just not coming to my mind)
- the manual edit transform I mentioned above would give direct access to placement and other details without using the mouse. whichever approach works for you