In my quest to get to the bottom of the annoying vibration / squeal on my rear brake I wondered whether a dab of copperslip was worth a go?

Yes, I've used it and it does help. Dampens the vibration which causes the squealing. You've got to be careful it doesn't contaminate the pads though.

Oh and it doesn't work with Hayes 9s. Nothing does.

It won't hurt – just a smearing though

If you haven't got any yet, Halfords sell it in small tubes, they call it 'copper grease' I think. for mtb that will last you years.

Tried on Juicy 3s with no luck, but you may do better. Small Halfords tube is way to go, and careful with it 'cause it gets EVERYWHERE and is impossible to shift. Wear old clothes. Under a bin bag.