My 2 cents:
It would theoretically be possible, right now, for memory hackers to read everything in memory and create a full 3rd party interface. It would require a fairly beefy machine to keep DF and the 3rd party GUI synced and perhaps some other voodoo, but how DF is laid out in memory is fairly well understood by folks who are a lot better at reverse engineering then I. Of course updates would probably break this client each time they happened, but everyone would know that like they know for all the OTHER 3rd party tools.
Would this be fundamentally different then using message passing between a DF-Server and DF-Client? I'm not saying that Toady should spend his time splitting DF into a server and client portion, but should he choose to do so it would most likely increase FPS since the program could be threaded, and it would also create the possibility for third party GUIs to take over the rendering side. There would be the stock-GUI for folks that love it, and there would be the possibility for people to create all sorts of wild and crazy interfaces should they take the time to do so. Of course updates would probably break these 3rd party GUIs as well from time to time, but everyone would know that's the way the cookie crumbles.
*shrugs* I just don't think the idea of 3rd party interfaces is abhorrent as some have made it out to be. Of course, if someone was to DEMAND that it happen, well that would be pretty silly, but I don't see anything wrong with an honest discussion of the idea.
(Note I didn't say jack-squat about open-sourcing the project. Toady's code belongs to him, and while folks can make suggestions he is ultimately responsible for the direction of the project.)