The files the registry is stored in are kept open by Windows while it is running for obvious reasons, so the only way to change anything in the registry is via the Win32 API; also, the registry is threadsafe, which would make things as file access much harder.

There is some documentation on the Windows 9x file format and the NT4 file format of user.dat, one part of the registry, out there, and I even wrote some partly Perl scripts to modify offline registry files, but I never got far, as I wanted to use it for changing the IP address of my notebook at bootup, but then changed to use DHCP anyway...