I've been writing tools in Maya for years using MEL and Python.  I'd consider myself an expert in custom window/gui design in Maya except for one area; modifying existing panels and editors.
Typically, I'm building tools that need totally custom UIs, so its customary for me to build them from scratch.  However, recently I've found myself wanting to add some additional functionality to the layer editor in Maya.  I've seen tutorials that explain how to do this, but now that I want to do it, I can't find any.
Links to tutorials or a brief code snippet to get me started would be great.  I just need to know how to find the layer editor/panel and, say, add a button or text field to it.
Have you tried searching ui item names in MEL files under maya installation directory? It should be one of the MEL scripts included, and from there you can just modify it.
How to modify existing panels in Maya using MEL or Python?