Now I know that I haven't been replying to thread lately, so I dicided to bring it back up on one of most recently made levels.
Here it is in the Overworld:
First the level was originally going to be called Pipe Maze, but I wanted to make it a cave later on, thus changing the name to Mine Shaft. Well anyway, the beginning of Pipe Maze is still there though.
As you can see, the place is a blue cave (much like the second level to "Mario X World DX" but the rom was locked so I had to make these GFX myself, they look like an almost exact replika, but if you manage to unlock that hack, then you'll you'll see a huge difference in my design.
So far so good, almost every sprite is in SP4. What's this? A pipe? Lets go inside.
Hmm, no bonus room, but a winged cloud is there.
As you see, dynamic sprites mess up the bottom half of SP4 GFX. Please thank BoingBoingSplat for the information. So I solved my little delima by putting it in a different room. Don't worry, you can use the same pipe to exit back into the main room.