I am a student aerospace engineering and I've learned a thing or two about material properties. Steel is about three times stronger than aluminium and three times heavier. (Depending on which types of course) The reason aluminium is mainly used to make airplanes is a combination of material properties, but the major reason is that because you need only one third of the thickness of the aluminium version if you use steel, you will have more buckling when the material is under compression, because it is so thin.
So you could use a stainless steel bash plate, and it could be one third of the thickness of a aluminium one, but make sure that it is thick enough to sufficiently resist denting, because you wouldn't want it to bend against your engine with every little stone.
The reason that most bash plates are made of aluminium are probably, weight reduction, corrosion resistance (stainless does not mean that it does not rust!), ease of production/manufacturing and looks.
Another alternative would be plastic bash plates, which can even be stronger than metal ones, provided the correct type of plastic is used. Another would be carbon/kevlar composite, which would be the best way to go, carbon is really very strong, but also brittle, when you combine it with kevlar, it is very strong and no longer brittle, and it is much lighter. The only downside is that it is slightly more difficult to repair (though not very difficult).
Wood on the other hand is an excellent material strength wise, and it is also very suitable for airplanes, the reason it is no longer used for that purpose is that it is more difficult to manufacture, it needs much more maintenance and is not that resistant against outside influences (warm/cold, fire, water etc.) This also applies for motorcycles of course.
So, you could use alloy or stainless steel if you'd want to without much difference in use. If you'd want something else (or better) go for a composite material and if you go for wood, do not expect it to last long.
Just my view on things,