when I spam N, sometimes I'll do a quick N-ENTER-N-ENTER combo, followed by another normally timed N... do some ultra quick spams, stop and wait for a response, normal spam, QUICKSPAM, it helps if you mix up your timing. I can get the Rate limit message to come up pretty easy now.
And don't worry, every time I've gotten the first xbox message, he'll ring up the xbox, then live, then something unrelated. Keep going! Another Rate limit should prompt the second message where he actually asks about the points.
And now it seems that typing "Rate limit?" is hit or miss, but when it works it works like a charm!