The huge amount of layers are beginning to become a bit annoying to work with.
How can I make it so that the player can't jump forward off of certain platforms?
Like invisible Dark Matter barriers, but forward-back, not left-right.

There are two ways you can do it:
First: Take a Gameplay Tweaker and disable the function that players are allowed to jump down on a layer before.
or
Second: Take a thin square of some material (its no matter which matter ) and put some invisible tweaker on it. Add a material tweaker to tweak if lights come through or not (change it to a static object in the object properties as well)

Hope I could help you

Greez
Dragon ~{}~

Thanks, I'll try these out tonight when I get home from school!
EDIT: Thanks, I didn't even know about the opacity tweaker! Much appreciated!

If you have DC Comics, there's a material just for that.

The old classic way is sticker panel set to 0 opacity lethalized with plasma.

or.. there's an old trick for that.

take some sticker panel, set to 0% opacity then add a danger tweaker with electricity or plasma setting. The game won't let your character jump into it from another layer.