Michael, I second that on the Shear(Skew) tool. Could work like the scale tools on the side panel. Doesn't have to be part of the on-View handlebar setup.

On the same token, an adjustable Taper tool as well.
I think both would work on a simple transform calculation.

Andy, check this out:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4382.1