Tip: Layer 3 backgrounds and tides disappear when you hit a message box, so plan your level around that. For example, you can add a sublevel without layer 3 at the start/midpoint, and include your message there.
Removed the sa1 tag because from my testing, it seems that this block isn't compatible with SA-1. The first reason being that the freeRAM is in bank $7F, which is inaccessible by the SA-1 chip (Snes9x doesn't consider this and allows $7F access illegally). The other reason is that, even after changing the freeRAM to SA-1 accessible RAM ($419D00 in my testing), the blocks do not disappear. While Mario's stood on a block it gets stuck in an endless loop of tile changing, severely overflowing NMI, and when he jumps off it, the block turns invisible, but doesn't actually disappear.
However, because this was accepted in the previous version (why?) and the mentioned changes for this update work as intended (reduced processing and wall kick compatibility), I'll accept it. In the next update it would be nice to see proper SA-1 compatibility.