They have documentation because it is possible to do it, but I guess the underlining point is, why do you want to do it, what is your reasoning? I think that would be something that could be explained a lot better.
Pulled this from the RIM document, "Placing the Blackberry Router in the DMZ"
A remote BlackBerry Router might enable further security options because the BlackBerry Router does not have encryption keys and therefore does not compromise the security of the BlackBerry Infrastructure if the BlackBerry Router itself is compromised. However, implementing the BlackBerry Router in the DMZ does not necessarily increase security."