Writing Lay-out Engines should be very rare. I'm not convinced writing a tutorial on that is a good investment of time. Maybe it's enough to add a sentence to the API docs: "If you plan to implement your own layout engine and need help with that, do hesitate the matplotlib developers via ...".

Yes it's hard to think of a short enough toy layout engine where this would be any better than reading the source code for our existing layout engines. Maybe someone has a simple example they can gin up, but this really isn't meant to be a user facing change, but rather something someone could write an add-on package around. Or a ready answer to "I dont like how constrained layout does X".

I'm closing this as not planned.

There's a minimal description in the LayoutEngine class ("To implement a custom LayoutEngine ..."). I believe people who are interested and able to write a layout engine are experienced enough to contact us if they need additional help.