TeXmacs is based on Guile and there is apparently already a Guile-based implementation of Emacs that is near feature complete.
Perhaps a Guile-based Emacs could somehow be embedded as an elisp-compatible interpreter "back-end" inside TeXmacs, much like how Mathematica is separated into a kernel (for computation) and a front-end (for input and rendering) that communicate with each other over a protocol called MathLink.
There is also DomTerm from Per Bothner, who perhaps coincidentally has also worked hard to produce his own Scheme-based version of Emacs (by way of the JVM).
One of the downsides of TeXmacs is the significant keystroke input latency caused by the extensive amount of page redrawing that is often required when editing "rich text." It's one of the main reasons that many still prefer latex-preview mode inside Emacs instead. Maybe latency would be less of an issue inside a graphics-aware terminal emulator like DomTerm.