From looking at neurosurgery applicant matches from the past 5 years or so, it is clear that there are few to no FMG's or DO's matching into this field. It would likely be much easier to work now to get into a U.S. allopathic medical school, then to struggle and match into neurosurgery with a foreign or D.O. degree. It certainly would be possible to do that, but you may have to spend a year or two extra anyways. It would be more reasonable to spend that time before entering medical schoool to get into an allopathic school.
As far as low-ranking schools, sure it may be more difficult, but I would venture to guess that it would be easier to get into neurosurgery from a 'low-ranking' allopathic school (and obviously what constitutes a low-ranking school is a matter of opinion in many cases) then it would be to do it with a D.O. or carribean degree.
Also realize that many (in fact, likely most) medical students change their mind on what they want to do as they go through medical school. Although neurosurgery might look attractive to you now, you may find a field that is even more enticing once you are in medical school, and perhaps choice of school will not matter as much.