First thing, if possible (and it may be too late in the game, since he's done the bones of the design before asking you), put a place to "be" at the bottom of the garden, even if it is just a bench, and way to get there. Besides the attraction of the destination, this gives the garden dimension and prevents you, and them, from seeing it as if it were a painting (especially not a painting lying on the ground). It is a space!

Second, all the normal horizontal rules apply for, say, controlling view corridors or creating privacy with upright plants or plant canopies. You are not ONLY looking down on this area (though you can get pretty fixated on that), but also across and through it.

So I guess this boils down to the fact that you needn't get bogged down by the slope. The space still has all its usual 3 dimensions. And if you create a way to walk through it, then you have all the usual opportunities to create pockets of close-up interest that, for example, rocks often provide. You can also do things that will look different from the bottom than from the top, so tucking a low plant under (behind) a tall one creates a totally different impact from below than from above.

All that said, many plants do look cool from above, in fact, in many cases it's their best angle. Grasses may be among those, but... not all of them.

Karin L