Let me just post about what I did last night so maybe someone can make better use of it.
The first thing I did was replace the basic right-facing platform with Battlefield's top platform. That way I could still use the left-facing platforms since they're the same. I imported all the textures so it wouldn't crash. They showed up but you couldn't stand on them or anything. The next thing I tried was replacing that piece's Type1 data with BF's Type1[2] (collosion data). I didn't think it would work, but I was sort of hoping that it would make the platform work... What it actually did was create an invisible Battlefield in the center of the custom stage. All I had to do was put one of the replaced platforms anywhere on the map and the BF would be there, including the main stage and the 3 platforms. I tried playing around with that but couldn't think of anything good.
Then I got an idea and replaced the BF data with Smashville's. By placing the square blocks under the stage at the part where the edges slant like \_____/ I was able to make a wall that you could ride up with your recovery. I was planning on creating Fountain of Dreams from Melee by putting rising platforms going through the floor, because since the SV model didn't take up any space on the stage editor, I could place pieces over its location and they would overlap in the game. Anyway, it almost worked, except that when you ran stright across it you would occasionally stop running and go to your walking animation, like when you were walking between 2 platforms. I tried adding a floor to it using other stage builder pieces, but due to the odd location of the invisible Smashville model, it was in between the grid blocks and the only piece that was at floor level was that half-platform thing. Trying to put multiple ones of those next to each other had the same problem where you couldn't run. So basically I had an almost-working FoD replica custom stage, except the floor sucked.
Now, the main point of this is that you CAN import models onto other pieces, but they will be extremely hard, if not impossible, to work with. However, I'm sure that at the rate we're going, some people will manage to create working ideas that implement creative uses of these model swaps.