Probably be a good idea to keep any plant or container to either side of each drain (she says stating the obvious
). The concrete looks like it would make for uneven footing, so the idea of using it as a ledge might be safer. If there is a section of available dirt between the base of the wall and the inside edge of the concrete, you could try trailing or cascading plants. They would cover the concrete as they spill over toward the patio. Depending on your growing zone and the amount of light, an example you could try is Cascading Rosemary (I have 'Irene' and it is a workhorse.) Short Clematis would be perfect, too.
If you don't mind annuals, something like Petunias or Sweet Potato Vine would be colorful and cover a good amount of space.
A few large containers could be spaced along the path between the in-ground perennials. Unless your winters are particularly cold, (thornless!)Roses, Irises, Daylilies, Clematis, a dwarf Pomegranate--each would fare well in a big pot.
Would you be interested in espalier plants? I know it is work, but the path end close to your house looks plenty tall enough to accommodate all kinds of trees or shrubs like Bay and Camellia, or Dwarf fruit trees. A climbing Rose with bright pink blooms like Zephirine Drouhin would be gorgeous against that dark stone, it smells wonderful and is thornless, too.
