It might work, but my experience is that the subject is in the shadow thrown by the lens (90 mm Tamron macro at full extension on Sony A-58).
Perhaps the paper will diffuse enough of the light down to remedy that, but then, how much light will you actually get on the subject?
I agree that the lens is likely to create a shadow over the subject using onboard flash unless the lighting is brought 'forward'.
Obviously not used the 'folded card' idea, but I find for macro this system http://www.nikonusa.com/en/nikon-pro...ht-system.html gets the light where you need it, before adding flags, reflectors and backgrounds which are still required in most cases. The other problem is that the onboard flash is much less controllable than individual units, that can be pre programmed to your subject. Just a tad(!) more expensive than a piece of card, but if something is worth doing, it is worth doing well.
Without having the restriction of the shadow of the lens, the solution of a remote flash placed away from the camera solves (partly) the problem of a proper illumination. Additionally it is a second trigger. In studio macro photography I `ve heard the use up to 3 flashes, but that`s not the case here, I thing that for field work you will find in You Tube many tubelike constructions that will take advantage of your flash light. About the diffusion they offer I think is fairly enough due to the distance the light travels through that constractions.
After all we can see the results in the photos that the authors provide.