Have a question for anyone in the know: while we’d been anxiously waiting for the decisions, D (international applicant) placed in several more regional-level and college-specific competitions (which, incidentally, takes care of her admittance to a university at home). Does it make any sense at this late date to try and inform admission committees? I believe the decisions, whatever they are, have already been made, but she really wants to leave no stone unturned.
bump!
I think decisions have probably already been made.
If your daughter is waitlisted anywhere, why don’t she try and write an interest letter and include her accomplishments? 
Most decisions are out, except UC Berkeley and some private schools. So you should have already heard back, if not, then you should wait until you have a reason to submit more info, otherwise, they do not even look at additional material sent in. For example, do as the above poster recommended if she gets waitlisted.