David wrote:

I should have said that my design will be concrete, with the rocks
mortared down in -- therefore no voids underneath. IMHO, that should
relieve much of the problem because the bottom drains would still be
pretty effective. (Of course not as effective as with no rocks at
all, but still reasonably effective.) AquaScape doesn't even use
bottom drains at all do they?

Here's another thought, one I've been pondering every since running across
a new substrate craze on some cichlid forums. There's apparently a rather
vocal group of cichlid keepers using 3M Color Quartz as a substrate in
their tanks. Here's the thing though, 3M Color Quartz was originally made
to mix into the plaster lining of pools and concrete of patios to give it
a color. See where I'm going here? It's obviously inert enough to work as
a substrate in an aquarium and it's meant to be mixed into things like
concrete. It comes in a wide variety of colors including natural tones of
greys and browns. Why not forgo all the worry about rocks and such and mix
this into the concrete to give it a rocky/sandy look. Anyone ever tried
this before on the water side of the pond? I've seen something like this
several times on the dry edges of a pond to form fake rocks out of the
concrete, but can't remember seeing any colored concrete in the actual
pond portions.

Here's 3M website on the stuff:
http://cms.3m.com/cms/US/en/2-125/cFikeFS/view.jhtml