Would using fused test leads rated at CAT III 1000V, CAT IV 600V, 10A allow use of an older non CAT III Rated meter on a jobsite as far as OSHA or NFPA 70E is concerned or would they still be considered only as supplemental protection?
In my opinion, no.
After all the device that is being connected to a source of voltage or current is effectively the meter, the test leads are only a wire link, fused or not.
What the CAT rating is used for, is safety against voltage surges and spikes, these inherently will not blow a fuse and can expose the user of such a meter to fire and explosion hazards.
Have a read of
this article from EC&M 
