There are not only proportional pools involved. Slush has a scoring method, bitcoinpool has a fine if you leave too early, etc. So it's more complicated to calculate. I guess if all pools would be proportional and have the same fee structure that approach would work.
Right, so you probably need to eliminate them from the rotation.
I agree. Most pools publish their cumulative hash rate. So there could be a limitation like "don't put in more than an additional 50% of their usual hashrate". That doesn't solve the problem though if you choose a pool which has no reputation and runs away with the BCN...
True, obviously you can't just do any old pool, but there seem to be a number of 10-100Gh/s, low fee, proportional pools right now that are solving blocks and paying out...