I really think you're going to want an external bluetooth unit. You really don't want to be hacking your brand new (expensive) phone by carving away the inside of the plastic back, or something equally risky.

Especially since, as Bratag pointed out, AR apps just aren't there yet in terms of maturity or usefulness.

It will be fun to build, and fun to goof around with, but maybe a BT unit stuck to your N900 with some velcro is your best bet right now...