This one ideally wants tilting bass drivers as well as tweeters. Speakercraft again do this in most of their ranges, there's the excellent AIM WIDE range which are perfect surround speakers - not because you can angle them but because they disperse in every direction!
http://www.speakercraft.com/#Product...7;20Wide%20One
Or maybe use some AIM LCR speakers where you tilt the whole unit in the wall.
http://www.speakercraft.com/#Product...M%20Wide%20One
Depending on aesthetics I do still like the idea of just boxing out a small section of the sloping ceiling to make the mounting position exactly match the non-sloping ceiling (or vice versa).