I would have to say that the 'group conscience' thing could logically trump such loosely worded traditions as the sixth, in this case. What I mean is this: this scenario you describe, it's not even close to being enough of an 'affiliation with outside group' that it would clearly run afoul of the tradition. This person would be arriving and speaking as an individual, not speaking 'on behalf' of some outside group.
A simple unanimous vote of approval at the NA meeting that it's okay for the AA member to chair at that NA meeting would certainly 'make it okay' in my book. If you can find an AA member to speak who is okay with showing up despite the (imho extremely minor) possibility that one of the members might vote 'No' to their sharing, then I think that scenario is perfectly within the 'confines', if you will, of the traditions. Just MHO, of course
