Yes, I had intended to do that but left it out.
Should it be a bitmask or just a button count? Buttons really have no standard
meaning so usually a button count is sufficient.
Some random thoughts:
* multitouch capabilities would be good to design in a mouse protocol for
2011, so having say 16 x/y pairs would be better
* on mac os at least scrolling is not done by pressing virtual buttons, but
by having a separate scroll interface that knows about velocity and such -
maybe worth adding that to the protocol from the beginning too.