I've never seen one that I would truly consider a must-have tool, which is understandable because so few really care about REG exports in the first place and even if someone developed one, they would likely leave out something important, or insert some silly annoyance anyway. But I'm always open to new ideas, so maybe someone will pop in here with something else to try.
I stopped looking and just set upon using a normal editor, UltraEdit, as my REG viewer/editor. This one is good because of the user customizable Syntax Color definitions ( plain ASCII file defining the pairs of syntax chars, delimiters and keywords ) that are responsible for how a given filetype appears in the GUI when opened. I find this approach "close enough for hacking work".
As noted above, INI files are very close ( but still different ) to REG files, so any INI file editor/viewer does a decent job. In fact my REG def for UltraEdit is very close to the INI def, which are both kept in the Wordfile.uew file used by UltraEdit ( I never did look to see if other users developed a better one, I'll have to check that out ).
If you use a bunch of different editors, you can experiment by just temporarily renaming a file's extension ( e.g., REG to INI ) to see how it appears in these different editors. There are lots of types to try ( REG INI BAT C CPP ASM ISS AUT AU3 PAS CMD HTM CSS XML PHP JS ... ). This approach let's you first determine the capability of an editor with respect to syntax recognition, and once you find one that you like, you can then go and duplicate that syntax, then tweak it up and use it specifically for registry exports.