Quote: "I can see that the shader thing would work but I don't see that there would be any advantage to doing that. I would need a full size depth bitmap for each level which is several screens in size. I use much less bitmap memory to have additional sprite sheets for each character."
Nope, no prerendered/backed depth maps ...just like a real depth map rendered each frame, the advantage would be to have that precize data additionally,
so you could use it for all sorts of effects and rendering one full HD texture each frame (or each change/movement whatever) isn't that expensive really.
I think running through for loops to set sprite scissor or whatever in AppGameKit Tier 1 is much much more expensive.
To draw the depth you can make an extra shader which dosn't need a texture, you only give it a depth uniform which you set anyway for the sprites.
I don't want to persuade you, as it looks like it works fine and why changing a running system, not ?
Anyhow I like the outline the most too and did you consider making the walls fade away or something.
Just throwing in some ideas really.