Wow, that's really awesome! I suggest you add some optionns for it to only hurt the player if he has a powerup and turn into a coin when a gray P-Switch is activated.
I also suggest you make an option to make it unaffected by gravity and, when directly placed on a slope, immediately adapt to it on level load, not just when it loads on-screen, otherwise it would look a little weird when it's placed at the start of the level.
One final option is to make it solid to both Mario and sprites, or either one of them. Keep it up!