Also interesting question for me. This is surely not the optimal solution (in case I'm understanding it correctly), maybe something like a workaround or help with research.
I've read your link and especially the IFilter interface and ISearchCatalogManager are good points to give a try to test it.http://msdn.microsoft.com/en-us/library/windows/desktop/dd940428%28v=vs.85%29.aspxhttp://msdn.microsoft.com/en-us/library/cc678933%28v=vs.85%29.aspx
I first tried it the manually way, so I've added a custom file type in Windows Registry \HKEY_CLASSES_ROOT\ , I've taken .tkz for example. You can see some sample settings in .txt file which contains Properties like this:
\HKEY_CLASSES_ROOT\.txt contains string (Standard)/(Default) with value txtfile,Content Type (text/plain) and perceived Type(text)
\HKEY_CLASSES_ROOT\.txt\OpenWithList -- to open .txt with named links (in my case Dreamweaver is linked there as string)
\HKEY_CLASSES_ROOT\.txt\OpenWithProgIds -- also "alternative" applications to open with (will be displayed in context menue when selecting a txt-file to open "Open with..."). In my case there are values from Visual Studio and Open Office.
\HEKY_CLASSES_ROOT\.txt\PersistandHandler --> Important because it holds the CLSID that represents a COM class and also a file name extension.http://msdn.microsoft.com/en-us/library/ms692491%28v=vs.85%29.aspx
\HEKY_CLASSES_ROOT\.txt\ShellNew -- not so important for our case.
okay, so I've created a custom extension, then I've quickly used Windows search and searched for my extension *.tkz. After searching for it I had a look into
Windows indexing options (run control.exe srchadmin.dll) and my custom file was successfully indexed in Advanced->File Types.
It's a bit dirty, but it would be an easy task to insert a new extension in registry with strings and values and doing a short search with windows after this. I've don't tried it yet to do this programmatically.
A second point I got interested to test is trying to use the Index-Management-Interfaces with method AddCatalog(). Maybe it would be doing our above step to search after our custom file above needless.http://msdn.microsoft.com/en-us/library/windows/desktop/ee872086%28v=vs.85%29.aspx
Hope it drives forward your researching in any way.
With Best Regards