The Phantom for instance, designed his new realtime based visibility equation on one of the ones I developed. Personally we discussed it and have both worked hard on the algo, and we have gone down different paths which suit our own style of engine. It is not really that realtime modifiable, but it does allow for a large percent of realtime modification. But the main thing to note is, that both these, although I haven't explained are actually rather simple tests, which can gain a fair bit of efficiency. Simply reordering code into the most efficient order, where the most things can be cached is also efficient.
Simplicity is often a great improvement.
That is not to say that all speed ups are simple. One of the speed ups I am studying at the moment is using parts of the other visibility system documented on my page, to significantly speed up testing, by calculating which polygons never have to be tested to other occluders, by splitting the world into 3d convexities. I could still dynamically modify parts of the world, either inserting convexities, or normal models, that wouldn't use the test, but we are talking a rather quick system here. The trick to designing a quick visibility system, is often to invent a reasonable one, and then work out generic optimisations, by either taking from other methods, or realising simplistic optimisations, or working out case optimisations, which can avoid large amounts of pre-processing. DirtyPunK PS Visit my site, and check out the two lame docs there, you might be able to see something that will help you with your own idea.