Another option for running a lisp (like language) on a small package is esp-lisp [1] running on ESP8622. The ESP8622 has a 32 bit processor running quite fast, and comes with 64K of RAM, so it's clearly not as challenging a platform to run lisp on as the Atmega used here. Its just a software project, though, and leaves the hardware part for others to design.

[1] https://github.com/yesco/esp-lisp