Hi there lookingforananswer,
Interesting. It makes sense. My own ex loved my cat and expressed himself differently when the cat was hanging around. Later one he got two cats himself and, from what I can tell, they are a source of uncomplicated joy in his life.
I think that's the key, the simple joy that dogs (more than cats) will have just to see you, is nothing at all like the complications of human relationships. Dogs are a source of constant validation for their keepers. And that's what pwBPD need - constant, uncomplicated, validation and no demands of emotional reciprocity. Also, the sense of fun dogs approach everything with, is just great to be around for anyone, BPD or no.
I don't think it would be a cure-all, and I know you're not suggesting that, but I can see that the devotion of a dog and the structure that caring for one would impose on life, could possibly be a good stabilizer.
Let's see what others say. woof