According to the docs, UglifyJS can mangle all property names except those on a provided reserved list. Is it possible to do it the other way, so only properties on provided list will be mangled?
If so, what options do I need to pass to uglify.minify(files, { ... })?
One (ugly) command-line approach to achieve the kind of whitelist you're looking for might be to use the regex option.
You can also use a regular expression to define which property names should be mangled. For example, --mangle-regex="/^_/" will only mangle property names that start with an underscore.
Otherwise, there is an open issue which provides the code you could use to modify uglify yourself which achieves what I believe you're looking for.