Actually it probably can be done as an 8 of 9 on 7 setup, with indexing, if you "hubbub" the rear derailleur cable mounting position. That mounting should bring the cable pull down to about 4.35mm which should work with a shortened 9s cassette. That said, I have found the 8s bar ends to be a little off in general with their indexing so the best thing to say is maybe.