Beagle, I've also 'heard' that the 787 is enough of a point design that it wouldn't make a good freighter either - I'm guessing that the main deck floor isn't strong enough and would require a major redesign (and unlike aluminum it's not a matter of simply increasing the gauge), but that's just an educated guess.
The same thing was said about the 777 back in the day. It was supposedly too highly optimized for the passenger mission that neither a freighter nor a tanker mod would be cost effective. But that was overcome and there are now well over a hundred 777 freighters in service. I'd guess that 787 will eventually follow a similar path. As for the floor beams, it would seem that adding a few layers of carbon composite would be equivalent to upping the gauge of an aluminum floor beam. But perhaps more significantly, the 787 uses different kind of seat tracks than past aircraft. Maybe that will become a driver for a freighter version.