Hi Jan,
Just had a nice browse through the headers, looking for something simular, setting the filestate for a scm plugin so it displays the correct state and hopefully correct icon.
Haven't completely figured it out, but if you know the filename, I think you might be able to get the cbEditor where the file is loaded and call reload on it. That might be a wise choice after doing something to the file through svn.
Another approach might be to get the projectfile and call SetFileState on it with the new state. Don't know if the editor gets an event and knows the changed state and will allow you to edit after that. As you have the source of C::B, just go and look what the different managers and their contained classes offer, you might get lucky and find the correct way to accomplish what you want.
Good luck, if I figure out how to do it, I'll let you know.