I don't think it'd be impossible, but the chances of somebody (who would have to be extremely talented with GScript and have past experience with this kind of thing) sitting down and spending a lot of their time on this is extremely low. 3D rendering has been attempted in the past on Graal using polygons, but making a full servers out of it would be extremely difficult.
Drakilor's planetary system was written in GS2 and was posted on the forums back in 2008, and he draws his 3D shapes using different polygons and some crazy math.
You can find it here.
I believe at one point Graal did have 3D capabilities but they were removed and the Graal 3D project was abandoned. Dusty's attempt at making a Mode7 system is something you might be interested in looking at, as it's possible that if something like this were to be fully released somewhere, somebody could sit down and make a small server using it, but it's not exactly 3D.
You can find it here.