if i'm understanding your question right, i think the answer your looking for is make the displacements a very basic shape with easy to work with 90 or 45 angles. Using displacement brushes that are like i dunno, 128 wide or 256 wide, and equally tall, basically small rectanglesr. And then when you block them together they make it appear as once large face. This allows the grid of the displacements to be small enough to capture finer details but they appear as though it's one giant displacement face. So to make sure your displacements stay connected, make sure to have them ALL selected when using your paint geometry tool, otherwise you'll botch the whole thing up and it might be easier to delete and restart at that point.
The map that showed me this most clearly that i can think of is badwater_basin. Open up the decompiled map and check out the displacement faces infront of blu's spawn cave. That demonstrates what i'm tryin to say perfectly. It's just a bunch of smaller rectangles that match up to make a very large cliff face.