Hey, that's a great idea. I have PLENTY of wild grapevines to use for this.
The clematis I have planted are pretty puny still, but I was checking them out yesterday and, short as the are, they're covered with buds. Healthy, if a bit on the small side. Their size is probably a good thing really, because the mailbox is getting replaced really soon (I have the new one already), and if they were much more established I'd be concerned about them being in the way.