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.
This definitely a creative idea for a block which could be used in quite a lot of places. You could have made it a little more user friendly, at least by adding more defines.
During the moderation, I added the sa-1 tag since the blocks themselves are actually SA-1 hybrids and I included an SA-1 compatible version of GPS's spawn_item_sprite routine. I added assertions to the Map16 variants of the block to make them impossible to insert without first patching the Bounce Block Unrestrictor patch.