Hey mjsdas, this topic comes up alot in regards to my tank, and I'm always happy to give my experience. Simply put, clams CAN be kept under PCs, but I would agree that probably over 95% of those put in PC tanks fail for many reasons.
The best way to go about it is to break away from the PC/VHO/halide debate, and look at it in terms of the intensity of light the animal will be receiving from your lighting AT IT'S LOCATION. The major reason I had clams surviving long term in my nano was that the PCs were just a couple of inches above the water (as opposed to six inches to a foot or more on larger tanks), and the clams were just a few inches down in the water column. Despite being under PCs, they were getting a nice bathing of light. Without my remote refugium the temerature on my tank would be in the high 80s.
Also keep in mind that clams take a while to wither away, although they sometimes appear healthy during this period. Success can only be claimed after many months and noticable growth. My oldest purple maxima was in my nano for about a year and a half and was purchased on Sept. 30th of 2002. My small squamosa surprisingly did very well on the sandbed in my nano, and after a while was getting semi-shaded by a soft coral nearby. Despite this it thrived and grew.
Another important factor is that many things kill clams, not just a lack in lighting. In this hobby it's so easy to blame lighting when something dies because it's easily quantified, while other causes, or combinations of causes, require some detective work and experience. I've been in this hobby for a long time, and this nano was very tough as it developed. It's a cliche, but smaller tanks are much tougher if you push the limits like I did.
What do I recommend? Take it slowly and gain experience. If your tank is succeeding I recommend getting a small piece of perhaps montipora digitata from a local buddy. This will be a good indicator of how your reef is doing and if this thrives you can move on to a small clam. Keep in mind you must discipline yourself to do this only when you are ready, and that may be years. My nano tested me and then some. You are much better off gaining experience with sps and clams on larger metal halide lit tanks as they offer more of a margin of error. With experience you could then think outside of the box. I own the four major tridacna clams (no gigas), and I'd recommend squamosas as a good starting clam. Derasas are an option but grow like roots.
One of my recent additions to the library is "Ultimate Marine Aquariums" my Michael Paletta and is a great read! It profiles many dream tanks, with many "outside the box" thinkers, including a 1500 gallon tank lit by 20 160 watt VHOs only that is 36 inches deep with clams on the sandbed and some 250 fishes. A must for the book collection. While much of their thinking is outside of the box, the people with those incredible tanks most likely had years of experience thinking inside the box.
Good luck and I hope that helps! Update us on your progress and above all have fun...
Johnny