Yep I agree it sounds like they should work
And yes the added wires are simply ground
And yet THEY DO NOT WORK.
I have actually tried it (Not just talking about it) and have tried several different round 80 wire cables (That yes work in a PC) and no they do not work on the Amiga.
NO IDEA WHY
When the OP gets his cables in, please reply back with whether they work or not.
(And if by some chance they do work for you please provide a link so we can buy them!)