Interesting, missed this thread...
I have had a similar intention for a couple years now. Where I live there is an old drum floating out in the inlet, chained off to 50m, each link about 6-8 x 3 inches. If one didn't look extremely closely at the right location, there is almost no sign that it is a chain. The thing is covered in life beyond my wildest dream of what would exist on a chain. A series of depth ranges result in different ecology. Here are photos of what I refer to:
http://www.quietdeep.com/scripts/site/event.php?qdEventPageNum=1
Now, it occurred to me that this could be repeated with purposeful intent. However, it is the only instance in the area of such a magnificent artificial reef. There are many other scraps and pillars around the area, but all of them have barely anything growing on them.
The following is specific to my climate but I am sure has parallels to all climates.
What I would say I have learned from the areas around here (chain, narrows, open coast, shallows, depth, kelp beds, etc...), is life flourishes with lots of water movement and good solid things that support anchoring in such places. Those are the fundamentals, but in no way guarantee a high chance that a reef will form. There are many places that appear to have all the same conditions as another, one having a flourishing diversity of life, and another not.
So, what encourages one area to flourish? I think part of it is just the luck of the draw, in the sense that life got a foothold for long enough prior to other carniverous creature or occurrences impeding it. The next most significant feature is interrupting water flow. In the short-term I think reefs grow more rapidly if the artificial reef is concentrated, without much surface area. This may allow eggs, sperm, and larva to have a higher chance of being "caught" and held long enough to become secured and continue their process of growth as a part of the reef. A good range of depth supports the reef growing from whichever depth tends to provide the most suitable environment for the reef to start. Many variables could dictate which depth is going to more easily support the growth, and once one area begins to blossom, that life attracts more life which will work its way up and down from that point.
This is why I think the chain has done so well. The links have holes in the center, making a great location for anchored life to grasp food funneling through. The depth is a huge range, supporting the complete diversity of reef inclined animals. The chain sits relatively far from shore with significant daily water movement with the tides. As well, carnivores of the reef life for the most part have no ability to reach the chain; they would have to be inclined to wander down to 50m, stumbling across the chain, and then proceeding to scale it. And further travelling up the chain, they have the possibility of encountering reef life that deters them, or eats them.
Cheers,
Tyler