You got to get enough access to get the cable into the box, anyway...

Pop a knockout out of the electrical panel, drill a hole up through the
floor from the crawlspace, and fish the wires through the stud bay. A pain
in the neck, but certainly doable. No worse than any other remodeling job.

But you can certainly tighten a standard connector from the inside,
but can't tighten the clamp portion itself on the cable unless can get
access to the outside...

I would think the cable clamp would work just as well in reverse? Put the
clamp portion inside the electrical panel? But, that would still leave the
problem of fastening the clamp itself to the box.

Unless, of course, I could find a clamp designed for this purpose.

Anthony