I'm not sure about BU, but I was late for UCLA (simply because the letter took 3 weeks to get to me, but you were supposed to respond within 2 weeks of the letter's date). I panicked and emailed them and the response was basically "no problem, we have your intention to remain on the waitlist marked down, we'll add the form to your file when it gets here".
If you are late, just email BU admissions and explain the situation. I doubt they are going to kick you off the waitlist because you are in another country!