Years ago I wrote/maintained a modal console-based email client. It was written with some UI primitives in C++ and the UI actually maintained and controlled by lua.

Viewing a list of folders? Lua. Viewing a list of messages? lua. Viewing a single message? Lua. Those were the three main modes and UI options.

All the keybindings, layout, colour setup, and similar was dynamic. It actually worked out really well. For comparison I just ran "wc -l" against he codebase: 60k lines. Combination of C++ and Lua, but mostly Lua.

Having good scope and good tests made such a thing fine to support. Mostly the pain was broken MIME messages and handling dealing with the external world - e.g. invoking gpg to handle decryption and encryption.

I'd work with big-lua again if I had the need, it's a fun language and very flexible.