The squeal on both bikes and cars seems to be related to pad material, although I've never seen a sure way of stopping it for many years.....sometimes it seems different methods as you describe can affect it but nobody seems certain.
Effectively you've got the pads to chatter on the disc surface, which sets the rasonance going through the disc (this is how I've had it explained to me) if you can prevent that surface interaction then the disc itself won't magnify it.
Make sure that you do have the right clearance to the disc if it's on the adjustable swing type calipers.
You could try this; rough the pads as you've previously done.....then when installed, run it around with the brake dragging to get it really hot......when it is, don't stop the bike while its' that hot but drive normally to let it cool......this is an old method of bedding in friction material and conditioning the surface of the pads original formed to get any of the unstable resins used in manufacture to burn out of the pad structure and can be surprisingly effective......assuming that was the cause.
It usually gives you a better initial "bite" and more linear response as well if it has any affect....not quite what the Triumph has though
