Well, as I understand it short stays make the front end of a bike come up more easily, however too short and it gets silly hard to bunny hop without looping out.
A long frame would in theory make it easier to hop as you would have more leverage, but with a long frame one needs more exagerated movements to get the same effect, this is due to the fact that the lever is longer so the displacement on it has to be larger, this means it will take longer to lift the front end.
In conclusion a short bike with shortish stays is best. This is because the bunnyhop is a fast explosive move, you need to make fast movements without the worry of looping out, but you also want the front end to come up reasonably quickly so a long bike to act as a large lever is out of the question.
High bb frames eh, well I have a wee theory on them, as you see if one makes the chainstays angle one gets shorter "effective" chainstay length (red line), I think.