Geese idea would work in one method: don't have all the races at war when the server starts. Just keep one race per side as "neutral" (ie. Admin says U can't play that race). Then, sometime during the dyna the Admin. has one chance per side to declare the "neutral" race active and allow players from any side to join. But, if you switch sides to play the "neutral" race gone war happy, then U cannot switch back. Admin. does not have to declare the neutral races of both sides active at the same time. Just one chance per side to have the neutral race declare war.
There are many historical analogy to this method. In the German campaign of 1813, Napoleon faced Russia and Prussia for the first 8 months of the year. Napoleon was trying to get a decisive victory over Russia and Prussia so that Austria would stay neutral. The Russians and Prussians successfully survived an Auterlitz-like knock-out defeat in 2 battles, and then Austria declared war on France in August. Napoleon, who had re-conquered all of Germany, then lost the Battle of Leipzig and was thrown out of Germany. France fell in April, 1814.