Exactly. An alternate layer (5, 6, 7, whatever) is given all the same properties as Layer 4, except for a Y speed of -1 and (I suggest) Tile Height. Create the visual appearance of the antigravity area in that layer, either from scratch or by copying it in from layer 4. Then reconstruct the mask of the area in layer 4 as best you can using invisible tiles, except the whole thing is upsidedown - I suggest copying from the antrigrav layer, line by line, except in reverse vertical order, and then converting to invisible by hand. For a foreground, employ layer 1 or 2 and give it the same settings as the first antigrav layer. Finally, lots of trial and error.
(Note however that very few tilesets have antigrav support as detailed as CrysilisV, which is why it was specially showcased at all; most of the time the best you'll get is your head partway into the ceiling like in Mike's first episode.)