Here`s a method that might work. Anyone who repairs video recorders has a sizeable collection of small allen keys. Get the nearest larger (either metric or non metric) - depending on what the original is, you might find that it`s only marginally bigger than the original hole and wont quite fit in. Freeze it with your trusty freezer spray and hopefully it will push in. let it warm up back to room temperature, and with a bit of luck, it just might grip enough to get the grub screw moving.
As the professor said, It`s a long shot, but it might just work.
Ron(UK)