The native webserver I built already works pretty well as a registry browser; it just can't make edits (at this time). I'm working on one with a bit more functionality (I've already gotten reading multi-strings supported, and I'd like to get editing working).
An on-device one would, of course, be nice. One option here would be to take the source code to a WP7 one and upgrade the project to WP8, then remove the COM interop used for WP7 homebrew and replace it with the C++/CX native component and .WINMD file for interfacing. Re-write the native interface layer to use the .WINMD (should be easy, actually, since there had to be a COM interop interface already; just replace that interface) and add ID_CAP_INTEROPSERVICES, and it *should* work.