I haven't tried clematis as a ground cover, but I would think to plant them in the middle of the slope and then they would grow all around. That's if you can maneuver yourself to get in there, maybe with mountain climbing gear ;^) Otherwise, I'd go to the bottom of the slope and plant them as high up as I could reach. I'm thinking that the clematis will grow upwards more towards the sun.

As for the planting hole, I would do it like you had planned, by digging out a hole and using good soil.

This is is what I would do anyway, maybe there's other suggestions that may work better. Anybody?

Good luck,
Laurie

I'm not familiar with the growth habits of crown vetch and lamium. Hops would probably do nicely, as well as the clematis. They would probably be best planted at the bottom of the slope. Gout weed, of course, spreads bit by bit, and the individual plants aren't that big, so they would need soil to spread into. It suspect it would need more soil than it sounds like you have. One thing with hops, at least the old-time ones I have: they take a few years to get established, but once you have them, they're almost impossible to kill. We have some planted by DH's grandmother that have grown in tall grass, been neglected, been mown... and they're still doing fine.
Connie