[This is preliminary documentation and subject to change.]
When Microsoft® Index Server 2.0 or later starts running, it reads a list of DLLs to register from a MultipleSZ value in HKLM\system\currentcontrolset\control\contentindex\DllsToRegister. For each DLL in this list, Microsoft Index Server calls DLLRegisterServer.
To make sure filters are always properly registered, add the complete path to the filter to this registry value when your filter is installed.