i think a bar attached to the winshield frame would only work if it was triangulated in some way; otherwise it would help the (for want of better terms) the 'foldin' of the car, but not the 'twisting' of the car. in my experience, most of the frmae movement seems to be of the twisting type (ie, backing out of a driveway crooked - as jaykay says, when you stiffen suspension, etc., you start to introduce flex elsewhere).
that engman bracket sounds interesting. go go gadget search engine skills ...
edit, here we go:
http://forums.pelicanparts.com/porsche-914-914-6-technical-forum/414638-pics-my-engman-kit-installed.html