I haven't, thinking that a new neck would look kinda funny on such a funky old guitar, but it might be worth a call.
Well, to get a D-18 neck off an existing, used guitar, there's going to have to be a reason that neck was removed in the first place. That, in and of itself, would be concerning to me. Why would someone do that? Is there something wrong with the neck? Second, the neck would have to be removed properly. If, say, in the case of a destroyed guitar, this may not have been the case. Lastly, I can't think of anyone better to install a new neck on a Martin than, well, Martin.
I'm a skeptic by nature. If someone has a D-18 neck laying around, there's a reason they have it laying around, and it's probably not favorable. I would trust Martin over anyone to do the work but, then again, I'm one of those guys who has no problem taking his 16 year old truck to the local dealer and paying them to work on it when it needs repairs.