From experience of having to add a full blown scripting language to two large domain-specific professional GUI applications in the 90's, and at the risk of upsetting the HN LISP crowd, you should go for something with a broader appeal/base than LISP and similar languages.

LISP and languages of the same ilk are extremely powerful and esthetically pleasing for CS-minded folks, but they are also and unfortunately rather off-putting for the wast majority of technical, non-CS folks (which IUUC is your target audience).

I'd go for Python, in spite of the lack of typing: easy to integrate, massive amount of libraries, very flexible.

An at the risk of committing blasphemy, I have to mention that the EDA folks seem to have a terrible addiction to TCL, which therefore makes it something you may want to consider.

My opinion of course, the best would be to poll your target audience.