I agree with the idea to sign the MPP and any ROPs just as we are about to declare. It seems as if 20 turns is able to be survived, according to the above posts. This will be glorious fun, too - I can't wait to see Germany fall to bits when multiple civs come to our aid....
BUT, since we ARE in a Democracy, we can't be too careful. Hospitals must be timed to be built only as the war is ending - imagine if we build 5-6 hospitals, our cities grow to size 14-15, and we lose several of our imported lux - we're screwed, and must take either the rep hit or divert badly needed war/science funds into entertainment. Hospitals are too risky right now. Maybe once a Colosseum or Cop Shop is up first and near the end of the 20 turns, but they could well mean a less than favourable end to the war for us.