One more EDA programmer reply, here... Tcl is super easy to embed as a shell for users, either in the terminal window the application is launched from or in a GUI text window. No doubt Tcl's embedability and historical connections to EDA come from Ousterhout's authorship of the Magic layout editor.
Lately I've been trying to embed Ruby the same way, and while I've succeeded, the process has been painful. The "event loop" is a particular challenge. So as far as I can tell Tcl is still leading in this regard. Can anyone chime in about Lua? I've heard some positive anecdotes.