STATUS

I mean, it was a stupidly complex solution that doesn't really work unless you're parallax mapping, but it basically involves rewriting the collision detection script for Tile B, then changing the values of those collisions through use of an in-game switch. Then, by having two different overlay images, you can switch one of them on or off to simulate height and depth, and place the necessary tiles where you need specific collision based on elevation.

Basically I made an engine-specific method of having four different collision settings: always passable, never passable, passable when switch is on but impassable when off, and impassable when switch is on but passable when off.

Like I said, it's stupidly complex. But it works, and that's all that matters for what I need it to do.

At the end of this month I'm going to submit a game page and demo for a new game I'm working on, and it'll showcase this exact functionality.