Actually the best time to do stuff like this is during down economic times. You get a lot more for your money when times are bad, which will probably end up saving the city millions on a project like this.
Plus: It sure would be nice to have the occasional Blazer "outdoor" game.
That would be killer, but it would almost have to be in the pre-season, I can't imagine trying to count on the weather to cooperate between October and April.