Don't forget to consider all of your light sources. Just because the sun is behind your subject, doesn't necessarily mean that they are 'back lit'. The open sky in front of them, is a light source. Anything in front of them that may be reflecting light, is a light source. So the sun isn't really your main/key light...it is just a hair/accent light.
But depending on the situation, the light on the front of the subject, may or may not be enough...which is where you can use your flash to add some fill.
I would suggest that the troubles/issues that mostly arise from this, don't necessarily have to do with reflectors or diffusers...it's more a matter of finding the right balance between flash and ambient exposure...or even just finding the right ambient exposure for the front of your subject (when backlit like that).
