installing a drip irrigation system above the succulent wall will keep them watered year round and on a timer to boot so it happen in the middle of the night and you won't see puddles (except from that pesky Seattle rain). 1/4" spaghetti tubing can be run pretty discreetly. Visit your local garden center for tips on irrigation and on constructing a vertical garden....and check out this Houzz link to inspirational photos:

http://www.houzz.com/photos/exterior/vertical-succulent-garden-