I can't imagine why one would want that. The DHE never captures 100% of the heat, and the shower mixer, thermostatic or not, does the rest of the control.
I looked at that too a while ago, not with any real intent. The showstopper there is that the recirc water must go down at least part of the drainage, where it can pick up all sorts of bugs.
It is actually. I expect their use will spread one day. No-one likes up front cost, but it pays its way and more.
NT