I am now looking at the Red programming language [1] for exactly this, but it does not have the libraries of C++ or Tcl. Still it is under 1 meg complete with libraries and it is homoiconic, cross-platform, and has a REPL and a systems language Red/System for going low level. The GUI and UI DSL and drawing DSL work on Windows and Mac for now, with Linux close behind.
[1] http://www.red-lang.org/p/about.html