I did find this, a leveling assembly that attaches to a flat plate. Manfrotto 438. The advantage is that nothing protrudes below the mounting surface, which in some applications is a big deal.
http://www.amazon.com/Manfrotto-438-Ball-Camera-Leveler/dp/B001A1VKPM/ref=c...
Downside is that a ball-based head cannot be swapped from the primary tripod to the unit with the leveling assembly, at best not quickly. Probably would require another head in the kit.
-> Ralph