This is the first i've heard of depth variances effecting heat transfer. I was under the impression that the whole slab just absorbed the heat and radiated it up. Very interesting.
My concrete contractor just hooked the rebar mesh with a rake and picked it up as they poured. My system has performed very well so far. (2 years)
Here's the company i bought my system from
www.radiantcompany.com
Maybe they could steer you in the right direction.
Wish i could be of more help, but i will tell you, you'll LOVE radiant heat.