well, to answer my own questions (in no particular order).
Chaiten is passable - police will make sure you don't stay overnight (people in santiago said it was closed, but everyone in futaleufu and around told me otherwise)
there is ceniza from the border to a liitle past Futa - from a light dusting to places (off the road) and inch or two. I tried to keep the chain clean, but you can only do so much.
the best place to leave the carretara austral is just north of cochrane via paso roballo - very pretty.
for anyone in villa o'higgins this summer, here's something to check out: 6km north of villa o'higgins a road goes to the rio meyer border crossing (48km to chilean checkpoint), where I was told that the river *had been* 20-30 cm deep and rideable, but was now 70cm deep and unpassable, except on horse and maybe in a unimog, which the argentine border police have one of. there is also some sort of foot path which has a very narrow pasarela (supposedly some motorcycle took a dive last summer). as it was getting late, I declined to go from the checkpoint to the river (3km) or walk from the river to the argentine checkpoint (12km). soooo... maybe someone can check it out, maybe pay for an extra plank, or maybe donate to the argentine border guards' recreational fund...