I didn't think a licence is the real problem, but the work permit is a huge pain. I flew in Panama as a training captain with TACA Regional, got a licence no worries (I hold licences from six countries, you pass the exam they give you the ticket, period). We were looking at Mexico full time, huge pain, but licencing not the stumbling block at that point. One of my former employers aircraft was down there (Mexico) for a while under Canadian registration with at least one Canadian pilot. If you check out the tails you'll see that many a Mexican airliner is registered in the US. Perhaps that has something to do with it. None the less, I flew in Central America and friends have flown down there with local licences; it has never been a problem (licencing, work permits are another story).