I wouldn't count on saver opening up at all, especially around Christmas. That's one of the peak seasons. If it were me, I wouldn't backtrack from SAT to IAD just to get a 777. If US is an option, I would go for that. Or connect through LAX/SFO. You'll have shorter flights (especially with a toddler) and a chance for an upgrade if you have one of the eligible UA credit cards.