Honestly, I think there's limited demand (if not need) for pathologists in the type of missions you're referring to. Obviously, a pathologist can provide important work in any treatment setting, but the fact is that most medical missions are mostly about helping the patients to simply survive - and that's not the strength of pathology (or psychiatrists, for that matter). However, we ARE best at finding out why they died, but again, that's not really high on the agenda in medical missions.
And trust me: Your clinical skills gets really rusty really fast as a pathologist. I haven't seen or heard a live, talking patient since mid-1999. OK, not entirely true, but get my point...
Also, I'm afraid that you WILL find it difficult to get a month off annually, both in residency and as an attending - in any specialty. Sure, the chiefs will probably think your plan is an admirable one. As long as you do it on your own time, not theirs.
By here's what's in demand (from
www.doctorswithoutborders.org)
"MSF is particularly seeking emergency medicine physicians, general practitioners, family practitioners, internists, pediatricians, obstetricians/gynecologists, and infectious-disease specialists."
Note that they're apparently not even looking very hard for surgeons, sleeper-docs, ENT's or Rads.