Making send public rather than the user busy callback fcn is an interesting idea.
After all, every pin in the interface gets manipulated to test the busy flag. An example of how that could be overridden with digitalWriteFast and another example with port commands would make it perhaps more approachable than the callback fcn is.
I don't know if anyone else has actually used the user defined busy routine and its kind of ugly, so I haven't been motivated to fiddle with it.
User callback fcns are the basis for the arduino's approach to user supplied interrupt routines, but the way it works here may actually be harder to understand.
I have thought I was done several times already. I think introducing the little benchmark (which after all, is not all that important; all the interfaces go faster than the eye can follow) has kept this fun and interesting much longer than it otherwise might have been. The project has served to teach me quite a lot about C++ and the arduino that I wouldn't have run across otherwise and provides a practical focus when I read about the language that is much more useful than the umpteenth discussion of circles, squares, and triangles inheriting from shape.