I was in an IB program for my last two years of high school, and yes, it is very writing intense. I would definitely talk to the school beforehand to find out what accommodations are possible.

Are you more concerned about the day-to-day writing (e.g. homework, exams in class, etc), or the official IB exams? Other than giving me a little bit of credit based on my exam scores, my college didn't care if I had an IB diploma or not, although *being* in the program was a definite plus when I applied.