I believe either the old dreamland site or the mylittleponycollector sight has a referal of brushes where you look for the brushes and figure out who came with them.
I thought of Lady Lovely locks too, I kinda want one too.(saw onw MIB for sale,I remember having one) this is a mystery indeed. I think I'll post this on my 80's forum and see if anyone there knows where its from.
I have/had one too. I'm not sure if it's still floating around here or not. I may have sold it with a fakie lot on eBay... ANYWAY... I believe that it belonged to one of my Sweetie Kitties, also made by Hasbro, but I'm only about 75% sure. Of course, with her now tangled, matted hair, I'm sure the mouse brush will never help... :? The link to a pic of the Kitties (and the Sweetie Pups) is below.