I am probably the least knowledgeable person on this subject, however, I'm doing a similar project and I'm happy to share with you how I intend to address it.

For me I have a tall block wall which I will use a smooth stucco to coat it and make it look like its made of clay (hopefully).

As far as grasses penetrating the gravel: I intend to remove / kill all grass / weeds etc before starting, then I will lay a layer of landscape fabric and cover that with the gravel. I have landscape fabric in other areas of the yard (previous owners of the house used it) and its great. I do get the occasional weed through where the overlap is or the edges of the fabric but otherwise its carefree.

Regarding drainage - I cant help you there, my soil is sand and I spend my time trying to retain water for the plants.

Best of luck. These garden walls are beautiful.

This is the texture I plan to use: http://www.thompsonbuildingmaterials.com/index2.html

Here is another wall pic. http://www.math.tohoku.ac.jp/~fujiwara/album/daitokuji/picture/PB130458.JPG

Thanks so much for the posting! That si brilliant, I should have thought of that. It is cheap and easy. I am actually builging the garden as a public space to encompase almost a full half acre. This is truly difficult, because I have never designed a japanese garden, it has to be rather cheap to construct, and it has to last. I think I may compact the soil, cover it in a dark plastic sheet, and put a drain in the middle under the gravel, so it would have a very slight slope at the bottom but be leveled out by the gravel.

Aren't these walls beautiful. It's been a few years since I read about this, but if memory serves, the covering on the original walls is some kind of mud which dries quite hard. It needs the little "roof" on top to keep the rain from damaging the walls. I think perhaps if you do a web search you might get the correct info.
And I have the same problem as taralynna, too much drainage and extremely limited water supply.
And the landscape fabric works like a charm. You might find that the black plastic sheet will work temporarily, but I doubt it will last. It will generate heat underneath, which will stimulate growth. One way or another, those pesky weeds will find a way through. The landscape fabric is far stronger.