I am assuming this is C. armandii? There is not a lot you can do - it is the nature of the plant itself to produce a bare base. All clematis (and most other vines as well) will do so over time if left unpruned. But since evergreen vines tend not to get pruned very frequently, if at all, this crops up more noticeably. Plus, the very large size of this vine encourages a more noticeable bare appearance.

The only thing I could suggest is to allow several long stems to develop but rather than training them upward, let them, cascade over the rest of the vine. This happened by accident with my very large armandii and the effect was exactly what you are looking for -- the foliage looked lush all the way to the base of the plant and it seemed to flower from the ground up.

The alternative is to plant something in front of the vine that will cover up the bareness.

thanks very much, G. both good suggestions on a coverup. it would be nice to bring some of the bloom down, too. we had 3 weeks of bloom but most of the scent was 30 feet in the air:)