Because I train in iajutsu, I agree that it would be nice to have some various Japanese martial arts demonstrations at anime cons, including Fanime.
But I'm not sure you could ever get a group from a dojo to do it? Would need manga/anime/video game fans who'd be willing to go to a convention...and I know among the dojo I go to, there's only been a couple anime fans. (Very sad fact, I think!)
And then probably the group would have to get special permissions from the con for showing up with swords/weapons at a con, let alone what might happen if some people/kids got a little too inspired by the demos and tried to practice on fellow congoers...yikes! ;p
(Side note: all my cosplay swords are safe-as-I-can-make-them wooden props that would never hold up to sparring.)
So until there's a martial arts demo at Fanime....
If you live in the Bay Area, in June in Santa Cruz at the Japanese Cultural Festival, there's usually at least one sword school doing a demonstration, along with a number of other martial arts. Also usually an awesome display of Kyudo or the Japanese bow, too.
A number of Obon festivals in the SF region down to Salinas and Monterey have kendo or iado sword demonstrations, my dojo only does one public demonstration a year, at our area Obon festival.