Hull Base

Is is possible or being done...to use large block of appropriate foam as a base, add appropriate stiffeners and backing plates and glass for say cat hulls or amas?

 

It is possible, surfboards are an example.

 

Styrofoam

Sure, and it makes sense for doing quicky prototypes. The problem is weight -
and you have to figure that out to see if its too much for your application.
I've been lucky to find an Architectural Foam place in Rockledge ,Fl that is willing to do all kinds of special stuff with 1,1.5 and 2lb expanded polystyrene.
This kind of foam is not suitable for a long lasting project because of issues of water absorbtion which I've only seen after 10 or so years. But you can't beat it for quick fabrication of something that floats-or that you hope floats.
You can only glass it with epoxy resin....

Look at this thread and the link provided-uses 6lb(approx) cove and bead foam strips: http://www.boatdesign.net/forums/sailboats/international-canoe-hull-design-40932.html#post509312 posts 14 and 15