Like a candle flame, all plant top growth begins in an upward direction. But then what happens after that depends on the plant structure. Climbing roses shoot out long, arching canes. The arches are supported at both ends by the earth. But if planted next to a retaining wall, without support beyond the wall they would arch over and hang down in "weeping" fashion. The clematis is going to spread in whatever direction it can, including down the wall. Even while growing downward, it's new foliage will be oriented in a upward facing direction, so it will look normal.

I grew climbing roses and clematis together once. Dumbest thing I have done in a while. When the climbing rose stems died and had to be pruned out , I practically destroyed my clematis every time. It did not work well for me.

Unlike planting a climber near the base of a suitable tree and letting it do its thing all plantings of climbing plants involving clearly defined architectural spaces such as arbors, trellises and walls require associated pruning and training on an annual basis. Otherwise a mess is liable to result. With 2 common mistakes being choosing kinds that are much too large growing for their intended spaces and failure to train from the start in order to develop an even furnishing. With instead them being allowed to grow immediately to the top of the support or surface and bunch up there.

How well a rose will drape over a wall varies with the particular kind. Clematis generally are inclined to go primarily up given the chance but I suppose many kinds planted at the top of your wall and not given any support would tumble over it. As a part of fanning out sideways in an attempt to reach something to climb on.

Again the success of this scheme would depend a lot on varieties being chosen that are not too large growing. Otherwise you will have some of the growth going down the wall and out over the sand. And same on the uphill side.

You could always plant non-vining types of clematis. These are more scramblers (sometimes even considered as groundcovers) and will spread and flow across the ground and drape willingly over the wall. And then plant a complimentary shrubby rose behind them.

Non-vining clematis would include those like Roguchi, Arabella, Sapphire Indigo, Durandii or any of the integrifolia or heracleifolia cultivars.

I believe you could choose a ground cover rose, which spreads out up to 6 ft. but stays about a foot tall. I am growing White Meidiland Rose right now, and it is a spectacular rose. Coral Drift and Apricot Drift Roses are considered ground cover as well.