Hi Mark, I changed my tyres at BMW in Osorno 3 weeks ago. They have an excellent range of tyres for albeit average prices (see motoaventura.cl). They had both Continental TKC's and Heidenau K60 Scouts + a few others. Speaking to a few Argentinian BMW riders most of them make their way to Chile to change tyres due to the hefty import taxes in Argentina.
I also paid a visit to the Honda dealer in Bariloche and they were very helpful when I was there but they didn't appear to stock tyres for big bikes, mainly trail/offroad bikes. Whatever you do make sure you lock your bike to a pole or in a garage in Bariloche, my GSA was stolen and recovered trashed just under 3 weeks ago, not the best way to end the ride...