I was having this problem for a long time, azaozz’s response helped me figure out what to do:
I couldn’t figure out how to add another css file to tinymce using the filter so I did a little digging and found that there is a file in public/wp-includes/js/tinymce/ called wordpress.css. I added the classes I wanted to this css file and they now show up in the post / page editor.
I don’t know how well this method will hold up to an upgrade (I have a feeling it wont), but I couldn’t find any information on the net about how to use the tiny_mce_before_init filter.
Ned
