Add contract/expand and blur functionality to the "set matte" effect

I often use one matte layer for many layers at once, but inverted.
Especially when keying.

So i use a tracked matte to separate the head/hair from the body, 
as it needs different keying settings.

This matte is then used as an alpha matte for the head and an inverted alpha matte for the body.
But then you run into problems with antialiasing, or visible seams, because of the different keying settings.

It would be nice to have the possibility to contract/expand and blur to have an overlap (expand matte) with a transition (blur).
THX