Emblematic of what's wrong with the Powershell (or perhaps MS as a whole) philosophy: you don't build this functionality into a shell, you build a file system module (you'd do it with FUSE if on Linux) that "mounts" the registry as a bunch of files. Then you could edit it in any shell or text editor, or even in a GUI file manager and GUI text editor if you wanted to.
I can see a lot more maintainability and portability of a implementation using FUSE to mount these systems as filesystems (so the abstraction is complete across the entire system instead of just in the command). Also the ease of having a abstraction which does not require special capabilities to be added to a program before it is useful is a reason that no system has properly supported these models before in this manner. On a UNIX system the same techniques could of been implemented sometime between the 70's and now to interact with environment variables and it has probably been implemented before but it has not gained mindshare as that approach is not scalable.