Marc, Michael,

The following steps are more for "skewing" or shearing objects out of perpendicular alignment, but could be used to get close in the opposite manner.
I'm not sure how to figure out what amounts in each step are needed to get an exact shearing angle, but it would be nice to add a handle or Transform tool to do this.
(hint hint)

You start out with your original object:


Rotate using the base line as reference: The accuracy of the Transform > Rotate tool is scary!


2D Scale a bit, either direction:


Rotate back to the origin line:


Scale in directions needed to get it back to relative proportion:



This is how it could be done:



You know, the more I use MoI's transform tools, the more I wish the "drawing" programs had them. ;-)