This seems like a really useful page: http://www.scantips.com/lights/flashbasics1c.html
It seems like you would lose ~ 2 ƒ-stops anyway by bouncing or off or diffusing through an umbrella. Ripstop nylon material is available at most fabric stores and could be hung on a t-bar (made from old tent poles or PVC pipe or whatever) atop a light stand for a larger diffusion source. You could even use a mirror a la Dean Collins and forgo the flash altogether and have auto-colorcorrected light.
Positioning your subject in the shade and building the lighting up is usually easier and more controllable. If suitable shade is not handy or doesn't offer the view you want consider making your own by positioning a ripstop nylon silk material between your subject and the sun. This alone can offer a beautiful light if the subjects face is lit satisfactorily, if not then add your flash or a reflector and possibly bring in a rim light. Or if you require a deeper shade the position a black material between the sun and your subject, and build up the lighting from there.