Control Panel applets can also be registered in "HKCU\Control Panel\MMCPL".
Well, technically, they would be registered in the [mmcpl] section of control.ini, which is then mapped in the registry (HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\IniFileMapping\control.ini) to the appropriate registry entry. However, as far as I can tell, Wine's profile string functions don't currently support INI file mapping (there's a patch on the bug tracker, but it was never accepted, it seems), so if any old application did try to write to the control.ini file, it wouldn't end up in the registry. I could add a check for CPL entries in control.ini if desired using the GetPrivateProfileSection function, and if/when Wine does support INI file mapping, the code wouldn't then need updating to point to the "right" place.