That would be the ideal notation for a diatonic harmonica player like me. I never really have an idea of what letter note I'm playing, unless I think about what key I'm in, and then I have to count. But I always know exactly where on the scale I am.

It's funny, before I read your comment I wrote a longwinded harmonica player's lament elsewhere in the thread, and I was thinking the same thing: A relative notation would be perfect.