Feelers--
I'm a believer in DSB for reefs, but I don't think its the way to go for ceph tanks. For one, even with an under-gravel plate halfway down, an octo would find his way under it, and the last thing he or she needs is a big gulp of ammonia, hydrogen sulfide or any other reduction products
I also wouldn't rely on a DSB for primary filtration. I think you should call a spade a spade and use a DSB for nitrate reduction.
The other thing is a reef tank is a slow, long-term nitrate generator--its a closer allegory to the natural environment. A ceph tank is very episodic: Lets say you get a small octopus, little filtration needed. It slowly grows and as it does your bacterial colony grows with it as they have more and more food available. In 6 months the octopus dies and all those bacteria starve to death. Essentially you're going to have irregular nitrate boom and bust cycles. I don't think this is well suited for the deep sand bed (and also keep in mind the dsb needs to cycle as well, remember those are bacteria that do the reducing, too).
I remember Colin once said that he thinks you should re-cycle between cephs, and I believe this is why.
If you are interested in experimenting with DSB, there is an easy way. If you go to ReefCentral and check out Anthony Calfo's forum, there's a thread about the "Deep Sand Bed in a Bucket." The idea is you plumb two small bulkheads through the top of a bucket, fill it with sand, and run water through with a powerhead. Flow has to be fast enough that detritus doesn't setttle. Easy to add, easy to remove.
Dan