Pat, I assume you mistyped and meant to say you wanted to put a clematis on the serviceberry instead of a trunk on it. Either way, yes I think that it should be doable. Generally speaking clematis do need something smaller than the trunk to grab onto. However once the young side branches that develop off the main branches of the clematis get long enough, either you can wind them around the serviceberry trunks or they will do it on their own. I have used this technique to get clematis to grab onto supports much larger than what is normally thought possible for clematis.

In terms of vigorousness of the varieties you have mentioned, Betty Corning and Alba Luxurians would be the most prolific bloomers and growers based on my experience with all four of the ones you are contemplating.

My brain must truly be fried from trying to solve the image posting issue. Yes, I meant to say I wanted to put a clematis on the serviceberry, not a trunk on the serviceberry!

Thanks for the input on getting the clematis up the trunks. That serviceberry is so twiggy and strong, I'm now wondering if it wouldn't have the stability for two clematises. Does that sound like overkill? Maybe the combination of Betty Corning and Venosa Violacea. Dan at Brushwood still has both. (I just put a VV in the front yard, but this is in the back.)

Pat