Slightly off-topic. I'm looking for a tutorial/article/project to create a toy (or even feature-full) emacs-like text editor from the ground up.

Using two languages: C and scheme (preferably R7RS-small).

I have quite a few resources to create a vi/vim like editor from scratch (vis, some python projects, etc) but non in the scheme+C arena along emacs lines.

Emacs project itself is a bad example of studying the architecture of such an editor because of its humongous codebase (~250 kloc C, ~1.2 Mloc e-lisp) unless someone has done a good write-up about how to get familiar with architecture and extensibility infrastructure.