heh, this is really hard (for me)..
i thought i had an approach figured out but it messes up in many cases:
it's fully parametric but..
if i were to just relax the mesh between the two outer curves (red) then it would bypass the mid-section of the middle curve.. once it stretches over the higher points of the middle curve, it then changes how the mid-section is stretched and it should now touch/anchor to the full curve.. instead of going through it as shown in the picture.
new approach is needed
maybe this weekend i'll give it another whirl.
i'd post what i came up with so far but it's a pretty good mess at the moment..