That's a lot of work for something that the dynamic linker on OS X supports out of the box. See, for example, <a href="http://toves.freeshell.org/interpose/" rel="nofollow">http://toves.freeshell.org/interpose/</a> or search for "OS X interpose" on your favorite search engine.
Interesting.. in the linux/unix world I've always intercepted functions with LD_PRELOAD. I'm guessing that's not an option on OS X?