Not yet. I've toyed with different ways of doing this, but haven't really decided on one that I really like. Maybe I should just go the road of other window managers that expose a scripting interface. Write a javascript file or something that defines a function that takes a list of windows and some other parameters and returns frames for the windows, binds to commands for modifying layout state, etc.

Would it be simpler to just define a couple "mirrored" variants of the main layouts?

Yeah, there's already an issue to track that (https://github.com/ianyh/Amethyst/issues/129), but in general it seems like people want more flexibility in defining how they want things to tile.

+1 for this question. I've got a dual head box hooked up to my machine, and I'd do just about anything for a window management solution that would let me divide up this 3840x1080 "virtual" display into two equal displays. Mostly so dialogs stop appearing in the split between displays.