I remember reading somewhere that each individual (default) level is actually a pre-rendered image (similar to how Cheapo works), rather than built by pieces like how Lemmix / Lemmini work. Of course, this doesn't hold true for custom levels.
I'm not overly familiar with the PSP version - does it give a black background when making custom levels, or is there some kind of graphical image? If it's the former, it should be very easy to extract the graphics; even if it has a graphical background, it might be possible to compare differences with and without a terrain piece on it.
I'm not sure how well an emulator handles the PSP version; but I know the PS2 version (which is virtually identical) works almost-flawlessly (just a tad laggy) on PCSX2.
On an unrelated note, welcome back to the forum! Long time no see!