I just tried it in an A2 head that has a McR20 which I shaved off 0.030" for focus with the Seoul LE. The top of the Cree metal ring stopped at the bottom of the reflector, and the back of the LE was flush with the end of the head. It worked fine, but the focus was off. The hotspot was broad, and had some dark artifacts. The Cree needs to be raised more into the reflector, but this might separate it from ground contact with the tube, depending on how much more it was threaded into the head.
The stock McR20S has a larger emitter opening that the McR20 with 0.030" shaved. As Nate mentioned, the Cree ring may be able to raise up into the reflector instead of bottoming out. Then you could focus it better simply by screwing the LE in until the point it touches the leads and shorts. You could then add a wire ring between the tube and LE to make contact. Or as John suggested, shim down the reflector. It might very well work, but the beam might not be as good as the A19 head.
I'm glad jch79 tried it with a Kroll. I don't have a Kroll tube that would work but it's nice to know the GDuP works well with it. I have an ROF multi-function and a FluPIC that I've tried with Krolls. The ROF works great, the FluPIC skips from the contact bounce and is frustrating.