It turns out it's not quite as trivial as it sounds.
The big missing item that would be required is adding a GFI circuit. The Hydra doesn't have one because the host EVSE is assumed to provide one. But having one is required by the spec (as well as being just downright prudent).
OpenEVSE includes a GFi, of course, and stealing their circuit for it would certainly be an option. The hard part is finding room on the board. The v2.2 board isn't very tight in terms of the component density, but that's deceptive - the trace density is rather high.
I could design a board that replaced the inlet management circuitry with a GFI. You certainly don't need both. But then I have two different boards - two entirely separate products, effectively. Given that I've only sold four Hydra boards so far (and two of those went to my employer), is it worth the bother?
I often feel like I'm sort of blogging in a corner by myself, unfortunately.