Although most clematis are not considered particularly drought tolerant, C. texensis may be a good choice for you. It is native to hot, dry areas of the southwestern US (Texas), it is quite hardy (zone 4), has a long bloom season and does grow readily from seed, although I have no suggestions of where to locate seed or as to how long after germination one could expect flowering. And I'd sure want to make sure it gets sufficient moisture until it is well established.

Clematis tend to become fodder for deer based on what I have heard from various friends who live in rural areas. Unless you have some way to protect them from deer, I would not suggest them. Also, it seems you are asking for too much from clematis--drought tolerant in full sun, fast growing from seed, and deer resistant. I think I would be looking at other options instead of clematis for this situation.