Latest developments...
I injected the foam yesterday evening and the results are good but not great. The essential problem is that the cured foam is not dense enough and compresses under pressure, which in turn allows the whole structure to deflect heavily. The good news is that the honeycomb structure is quite strong even given this. I layed a 1/16" plastic sheet on top of the honeycomb and inched my (210 lbs) way out to the center of the triangle (see image below) and although the surface deflected it held up and recovered nicely after I stepped off.
The foam, in addition to being structurally inadequate, is very hard to control. I'm looking for alternative materials to use in the interstices between the PVC sections. I'm leaning towards trying epoxy next but am open to other suggestions. An ideal material would be inexpensive, easy to apply, and cure really hard.
BTW, I also learned not to over tighten the wire ties before injecting the foam as it makes it very hard to form the honeycomb into curves.
http://dws.editme.com/files/JanFeb2008/deftest1.jpg
http://dws.editme.com/files/JanFeb2008/deftest2.jpg
http://dws.editme.com/files/JanFeb2008/foam%20closeup.jpg