Actually - this sort of topic has come up before - lots of info on this thread...
http://www.bikeforums.net/showthread.php?t=396754

If it were me I'd look into (rarish) F1 BMX frames, and 'Tall' adult sized frames like the Mongoose moosgoose as a starting point - the geometry might work better without need for a really long seatpost.

F1 BMX:



Another:





Moosegoose- look at the height of the top tube compared to a 'normal' bmx...


Moosegoose:


...compared to 'Normal'