Let me preface this by saying I am not a professional like many of these guys. My knowledge and experience comes from renovating/remodeling 80+ yr old homes -- our own home(s) as well as friend's and family's homes. I've never been paid for this kind of work, but I'm not a fool and learn how to do things before jumping in.
As you already know, the preferred method (rather the 'required' method by code) is to vent to the outside. Several have already pointed out the risks of moisture entering into the cavity between your joists. Based on your initial post I assume that you already recognized those concerns. Only you can be the judge of how much moisture may be vented into the cavity and how often it might occur. Based on your comments, I would not view moisture as a likely potential problem -- possible? Yes. Likely? Probably not assuming the facts that you present. Incidental humidity created from one's breathing or a running sink faucet are highly unlikely to be problematic for you unless the 1/2 bath is EXTREMELY small. I typically-sized half bath is going to have sufficient air volume that the humidity created by normal handwashing, flushing, breathing will be negligible, IMO and experience. Now, if someone runs the faucet with steaming hot water for a healthy length of time, then, yes, excess moisture could (probably would) be created in an amount sufficient to cause a problem. However, if you or someone else accidentally vented that moist air into the cavity you could simply leave the fan on for an hour or so and the moving air within the cavity should be sufficient to lower the humidity of the initial moist, vented air and prevent any problems, particularly if the cavity between the joists is of fairly good size. (It should probably be noted that I am assuming that there is no insulation inside the joist cavity -- if there is insulation I would not vent into it no matter what as it is just too easy for problems to develop.)
The only real concern I would have, if I were in your shoes, is nelie's comments about fire safety. Although it's not likely that your house would ever be involved in a fire, it is possible that the vent could contribute to smoke/fire damage if such occured. I would not be concerned about the possibility of denied coverage though -- failure to follow code by the homeowner is not a valid basis for denial of coverage, at least it's not under Missouri law where I live. Nelie's point is well taken though.
Ultimately, it is your house and you have to assess the risk/reward of venting or not venting. I can tell you that I have seen a fair number of incorrectly vented fans in these old houses and have yet to see a problem as a result. I wouldn't foresee a problem based on your description, but there are no guarantees.
If you do decide to vent into the cavity, I would just be diligent about running the fan longer than normal -- the increased and sustained air flow within the cavity will help prevent any problems with regard to moisture.
Hope that helps -- take it FWIW -- which isn't much considering the cost for the advice. :>)
good luck, let us know what you decide to do.