One styling option would be to use a triple triangle frame like that of the Highpath' SpeedLight or "Big Blue" bikes. This would be more retro whilst achieving the low step over height required. It is also a very elegant and lightweight solution from a structural point of view.
Having kept quiet on this thread so far, I feel it time to stick my head up...
I think DWS would raise an eyebrow slightly if he considered "Big Blue" to be a styling exercise. Even with his sculptural background.
The Highpath Highlight design is a brilliant structure, which is a modification of a 1800's ladies bike frame, with some additional sparring.
It offers a tall headtube with excellent frontal impact strength and very good mechanical efficiency. It's this design I started doodling with, when I found a factory that was making these - http://www.sebikes.com/quadanglelooptail24frame.php
- , when I was in Taiwan last week.
Geometrically, it would be quite amazing if that frame Geoff was optimal, being a modified Banshee dirt jump frame, modified with different dropouts, different forks, different sized wheels. There's a fair bit you can do with the modifications that have been done there, but I think we'd all agree that perhaps the BB is a *little* bit too high and perhaps the head angle is a *little* bit steep.