What if, instead of having each game be 12 days, the games would last four weeks (or just under) but with a two-week overlap? That way, once the pipeline gets going, games would still be finished every two weeks, but people who go on vacation or get busy with something else for a while wouldn't be effectively knocked out of competitition. Movement up or down would have to be handled a little differently since a new tournament would start two weeks before the last game of the previous one is finished, but reasonable workarounds could be found. For example, the person in first place after three games might be forced up, and also the winner of the previous tournament if that was a different person from the one who had been in first after three games.
Overlapped games with a longer "time window" for each would also make it a lot easier to work in a large-map game, perhaps in a "Small, Large, Tiny, Standard" pattern. Players who aren't finished on the large map yet after two weeks could spend a little longer on it and still have time to play the tiny-map game.
In regard to whether movement up or down should be mandatory or voluntary, consider the implications of each. If I struggle at a higher level instead of playing at a lower level where I would be more competitive, I am not harming anyone else in any way. Therefore, there is no compelling reason why I should be forced to move down against my will.
On the other hand, if I stay in a lower bracket when my skill level is such that I really belong in a higher one, I am taking away a big chunk of opportunity for players who do belong in the lower bracket to get top finishes. It therefore makes sense to force the person who finishes first in a lower bracket to move up and make room for others. (Below that, forcing people up deprives them of their chance to be first now that last time's winner is in the next bracket up, so the situation is less clear.)
One last thing: I think cultural victory definitely SHOULD be turned off when the goal is some other kind of fiinish. Expecting people to avoid conquering so much territory that they achieve domination or conquest before they win the space race does not harm the realism of the game, since there is nothing at all unrealistic about a nation's being content with ruling less than the entire planet. But the idea of having to deliberately hold back on improvements that make for a better empire for fear of building up too much culture too quickly seems ludicrous. I have no real interest in a Civilization game where one of the goals is to avoid getting TOO civilized.
I'm not sure yet whether I want to commit as much time as something like this tournament would take. I'm a micromanager (albeit not focused so much on score as on other aspects of building a great and glorious nation), and that means each game tends to take a big chunk of time. I'm also not sure whether I'd go for the middle or top division if I do play; my skill level is such that I pretty definitely do belong on Emperor level, but I'm almost equally sure I don't belong on Deity level at present (especially given my distaste for exploiting every available weakness in the rules). And going down in glorious defeat isn't exactly my idea of fun.
As I've said before elsewhere, I play tournament games mostly for fun. I enjoy seeing how my results compare with how others do on the same basic game (especially when I do well

), but I have no interest in sacrificing the fun factor of the game just to play on a higher level or do a bit better in tournament rankings.
Nathan