I think that they only show up on Blood Moon which means they will be gpsing to you and you would only be able to get them close enough to someone else's fort walls if you had already breached them somehow. I'd love to see a video of someone actually pulling it off because I can't think of how it could be accomplished unless the person you are trying to raid has a really bad defensive structure anyway or didn't use a landclaim.
Now if demolishers were to spawn normally on a regular night and you threw a rock to get them to run over there and then shoot their detonator that would be a different matter but on blood moon they aren't going to go for a thrown rock...