If you were to construct a support system such as you'd use for grape vines then tht would have plenty of capacity for Clematis growing.

The Clematis varieties with serious weight are, for example, C armandii, montana and buchananii where the stems get to be as thick as an adult wrist and they easily pass fifteen feet in height.

I think I'd go for rails rather than trellis, too. When the stems get in behind trellis they can force it apart.

Also remember that you could be doing an annual prune on the jackmanii Clematis so there's plenty of new growth to provide flowers.

At 20+ feet long - sounds like you have a colour combination in mind. Hope it turned out just as you hoped.