I was trying to find a way to figure out what changes a particular dll's DllRegisterServer make without actually changing the system. Then I found this:
The RegOverridePredefKey function is intended for software installation programs. It allows them to remap a predefined key, load a DLL component that will be installed on the system, call an entry point in the DLL, and examine the changes to the registry that the component attempted to make. The installation program can then write those changes to the locations intended by the DLL, or make changes to the data before writing it.
For example, consider an installation program that installs an ActiveX control as part of an application installation. The installation program needs to call the control's DllRegisterServer entry point to enable the control to register itself. Before this call, the installation program can call RegOverridePredefKey to remap HKEY_CLASSES_ROOT to a temporary key such as HKEY_CURRENT_USER\TemporaryInstall\DllRegistration. It then calls DllRegisterServer, which causes the ActiveX control to write its registry entries to the temporary key. The installation program then calls RegOverridePredefKey again to restore the original mapping of HKEY_CLASSES_ROOT. The installation program can modify the keys written to the temporary key, if necessary, before copying them to the original HKEY_CLASSES_ROOT.
This is such a blessing.