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
That could very well be the case. I suppose a non CAT rated meter would likely have a low impedance design as well. Some of the statements in that article still have me wondering though.
Would it seem logical that the test leads being fused at 10A would be the first in line to see the increased current and could potentially open faster than the internal protection in the meter would, so in essence preventing any further damage?
The more I read on the subject, the more it seems that these safety ratings are contemplating complete and absolute blissful ignorance on the part of the person using the test equipment. So much so, it appears that they group test equipment users into the in the realm of unqualified amateur. Apparently these days, it really is too much to ask for an individual to be competent in the things he does and equipment he works with on a daily basis.