If your config is that complex, you might be better served by JSON - or the full JS or, say, Lua, for that matter. Because what you are talking about looks more like (interpreted) code than a config.
It's under- and mis-used, but XSD helps to validate config files and provide some guidance on the structure. I know JSON has a schema in draft, is it used much?
An example of XML config we used at my workplace was a processing pipeline with various modules and options/parameters encoded for each phase (some optional) of the pipeline. So in a sense it was configuration that resulted in executing code modules, not so much your standard options.
