Assuming that the input will be between 0 and 255, we're looking for 127 as off, 255 as fully forward and 0 as full reverse, so how do we achieve that?
It varies from manufacturers. Some ESC allow reverse, or disabled reverse and allow breaks instead, or some combination (break for 5 seconds, then reverse).
for example HobbyKing has a bunch of programmers to make changing it easier. In most cases the same thing can be done with radio controls, although it generally is a bit more challenging.
I looked at your page a bit but did not see a reference to which speed control (ESC) you are going to use. If you have not chosen one yet, be aware that they usually come with instructions for programming reverse handling, timing etc.
Your assumptions about input seems reasonable, and is what I think you will find once you get going.
While my answer does not address reprogramming ESCs, which is the actual question asked, it does turn the motor the opposite direction. And it was hard for me to find a good answer on the web. Google searching can end up here, so I'm posting what I think is helpful.
For my ESCs, I have three wires coming out, and to reverse the direction of the motor, all I have to do is swap any two wires that go from the ESC to the motor. It worked for me. YMMV.