you could even do the staggered-days thing for EU/US, too (i.e. when EU is on wed-fri-sat, US is on thu-sat-sun, then the other way round). that way, there's less overlap and so less chance of ending up with two half-filled servers, with the exception of saturday, which is the strongest in population probably anyway, reducing the half-filled server probability.
or would that fragment too much? I'm not sure, this depends on if US/EU people will still play on the other "side's" higher-lag server, or decide to stay away until "their" event comes up.