guessing you would need quite soft foam, otherwise it wouldn't deform

The foams they use for helmets are pretty hard. They won't deform until you
hit a threshhold. You don't want something that will compress under normal
loads.

Agreed.

I imagine you would want something that would deform around (for the sake
of argument) 10-20Gs. One's back probably has 10-20x the surface area of
one's foot, so my worry is that if you used a foam that was soft enough to
protect your back, it would be easy to put your foot or elbow through it.

Another idea was putting a big chunk of foam at the front of the aircraft
to slow the deacceleration if you hit something frontways. Say you have to

We could be talking about a lot of energy. Might require a lot of foam.

airframe may break. Perhaps a cubic foot of harder, crushable foam in the
nosecone could reduce peak loads on the airframe and spread them more

Where in the nose would you put it?

Behind the canard - in my dreams.

I was thinking of maybe something behind
the instrument panel, maybe attached to the firewall. Speaking of fire....I
think I have an old bicycle helmet laying around. Maybe I'll put a match to
it.

Good point.

Anyway, I'm sure there's data available on foams currently in use.

I have heard that the racing drivers use alu honeycomb. That sounds like
it might be seriously expensive.

AC