Hi Lyor
Having watched this thread for a little while, without anything serious coming up to help you studies, I wondered if a lawn tractor grass collector could form the basis of a simple baler.
Have a look at this link to see how such a device works:-
http://www.countax.com/pgc.htmI have one of these and the rotating brush does a good job of sweeping up the cut grass or straw. The box could then be modified so that you could drop a flap down onto the collected material before tying it into a small bale.
The collector clips onto the back of the lawn tractor using something similar to a 3 point linkage. This allows power to be transferred using a rubber belt. A towed collector would need a separate engine or a Cardan shaft from a power take off.
This would not give you a large or tightly compressed 'brick' of straw but it would be enough to store some in a small stack.
The lawn tractor I use has a 14hp engine and little power is required to drive this collector. I think it would require 1-2hp possibly less. The clever part would be a system for tying the bale so that it keep together in a rectangular shape.
The brush can pick up weeds and similar coarse material as well as fine grass clippings. So the machine would be quite versatile.