The reason each loop has its own block, pump, and radiator is for better cooling on top of the single loop of cooling. It might hurt if it was all crammed into a small case, but I'm not looking to shove it into a small case. I'm going to expand the case to make this all fit nice and neatly under the main case, which will still have air cooling for the RAM and North bridge passive coolers.
Better cooling is not necessarily achieved with separate loops. Water blocks and radiators perform better with greater flow - examples can be found on Swiftech's website, where they publish test data indicating the performance increases you can expect with greater flow rate. As long as you have enough radiator capacity in your loop, it doesn't matter how many things you are trying to cool - more flow = more performance.
I'm not saying your idea won't work, but by splitting things up you could be wasting pump power. If all of the loops are going to share a common reservoir, you're essentially cooling all the components with the same loop anyway, so why you would want to decrease efficiency (flow) through some of the blocks and not have any kind of redundancy is beyond me. If you're going to spend time building a custom case, and money buying loads of pumps and radiators, IMHO you should try to get the most out of it.
If you want everything to be water cooled, I can understand having 2 loops: One for the important stuff (CPU and GPUs) and another for the rest of your junk (RAM, NB, HDDs, etc.) But even then, in order to deliver the best cooling to the CPU and GPUs, you'd want to keep the liquids separate
. Sharing a reservoir would just dump more heat into the important loop.
One other suggestion: get a large case. If you're considering buying 3 or 4 pumps and 3 or 4 radiators and who knows how many waterblocks to cool God-knows-what, I suggest not using some sub-$100 TT case. Take a look at Lian-Li, particularly the G70 or V2000 series.