Well if you weren't in zone 5 the first thing that popped into my mind would be of the many montana clematis. They get huge, need little pruning, and there are many varieties available in different colors and fragrances. However, the montanas are iffy at best in zone 5. There are some people who live in microclimates in zone 5 where the montanas not only survive but flourish. Don't know if they would act accordingly in your garden, but it would be worth a try.

SAC is such a mundane clematis and can be a rampant seeder given the proper conditions that I would shy away from it. There are just too many other clematis out there in my opinion that deserve garden space compared to it.

You could always grow a type III and not prune it however mother nature might just do the pruning for you depending on your winters. If I were going to try and cover something with a type III and not attempt to prune it then I would go for Betty Corning because it is one vigorous clematis and its blooms are fragrant.

Another type III clematis that might work for you would be Triternata Rubromarginata. Again it has blooms that are fragant and is very vigorous once established.

Just realize that any type III clematis, including SAC, would be cut very close to the ground each season. Doing so in the dead of winter would allow the vines you couldn't pull out of the higher branches a chance to blow out of the branches over the course of the winter.

One thing to consider is that the tree is eventually going to rot and fall over regardless of what you end up planting on it.

Thanks so much for all that information. I think you suggestion of Triternata Rubromarginata would work really well. Now I just need to find it.

Diana, Debbie at Silver Star Vinery still has it listed as available on her website. Dan at Brushwood had it but is sold out.

Here is a link that might be useful: Silver Star Vinery