I've got some experience working with similar configs using a DSL which look similar to a restricted Lisp.

> Configuration file should be passive data, not a programming language

Some interesting config file formats now represent nested lists. That's enough for S expressions representation.

It's arguable what configuration process does. Especially if it's (safely) not Turing-complete. Setting "passive" values of some parameters in today's configs could lead to quite complex changes in the configured program behavior.

> and as easy to process in variety of languages as possible.

S expressions are parsed in many languages e.g. here - http://rosettacode.org/wiki/S-Expressions . Don't see a problem with processing.