I've got a suit made in the 290 gram fox flannel. It's not terrible, but it's a less than optimal choice for trousers. In my experience all flannel has some give to it over time, but it's more pronounced and happens faster in the 290 and as a result ive ended up with pants that don't hold their shape as much as I'd like.
I think it would be awesome. I'm considering something similar right now and fresco is on my list of possibilities. One note if you haven't used minnis fresco before. I find the fresco to be unusual in that the fabric really has no give to it. I can't think of another wool that is quite as rigid.