For anyone considering building their own independent competitive registry (there's been quite a bit of talk about it here), it would be nearly impossible. Neither NGC nor PCGS offer an API for their certification look-up tools, so you couldn't authenticate that any given coin is the grade the user says it is. Meaning there is a lot of room for abuse (I create a collection of double eagles and say they're all MS 70, who can prove me wrong? Or I create a type set and add all of Bill's certification numbers in my slots, will your software stop me? Will you offer enough customer support to address Bill's concerns when he writes to you about it?).
At best you could hire someone to manually look up the certification numbers and approve listings as they come in, but that would be a full time job (and the registry definitely would not produce enough traffic to pay for itself, let alone hiring someone on salary). And then you'd have to add in ANACS and ICG, again, no API tool for certification look ups (and remember, everyone wants the registry to be inclusive so the top four at a minimum should be included).
Then there are the competitive sets themselves. How do you go about building those with everyone wanting so many specialized sets, and people arguing over which coins should be included and which shouldn't. Me personally, I'd use some kind of third-party standard, like the Red Book. If it's listed in the Red Book, it's in the competitive set. So if you want your highly-specialized variety in the competitive set, petition the Red Book, not the registry. But that only works for the US. There is no Red Book for Canadian coins, or Australia's mint, etc.
Then the scoring... population reports are changing every day, and again, no API tool. So how do you determine the score for any coin in any grade? PCGS's approach of grade x multiplier for rarity is a bit cleaner than NGC's algo, but still requires you to assign a rarity multiplier to every single coin.
Taking the above into consideration the only real choice is to build a showcase site, non-competitive, that lets you build custom sets. But CollectiveCoin and even Photobucket already allow you to do this, so I'm not sure there's a need for yet another site.
All that to say, I own registrysets.com but I'm not sure there's anything that can be done with it.