Just for the point of arguing, you're talking about direct string vibration. But the entire instrument vibrates (well, assuming we're talking about a good instrument hehe) sympathetically with each note played, right? As per your example, you could indeed wrap some foam around the part of the string behind a fretted note and notice no tonal variation (totally agree with you there). But could you replace the entire nut with foam and still hear no difference? In the overall timbre of the instrument?
That aside (and I realise I'm using highly exagerrated examples and being an e-shit) I really question hearing a sonic difference between corian and bone though. Corian's pretty close to bone in my experience? It's good enough for Martin... VP if you could record this I'd be most interested (note sure how you will take out all the other variables though). Saddles, maybe, nut, dunno.