Yeah, I had the same question after I started seeing inconsistent behavior trying to tag everything in iTunes. I've now installed about 3 or 4 tagging tools... They seem to be either a) comprehensive or b) fast - but I haven't found one that's both. :sigh!:
I would take a look at Mp3tag (http://www.mp3tag.de/en/)
Select a file and then you have to click the "Extended Tags" button on the menu bar. It will let you add the TITLESORT tag. I have confirmed that iTunes correctly reads the tag set by Mp3tag.