Use "ifconfig" to conditionally add CSS assets
It seems like it currently is not possible looking at the head.xsd
definition in the develop branch:
https://github.com/magento/magento2/blob/develop/lib/internal/Magento/Framework/View/Layout/etc/head.xsd#L9
you could do something like the Require Js Module: https://github.com/magento/magento2/blob/develop/app/code/Magento/RequireJs/Block/Html/Head/Config.php
Here, the require config and mixins are inserted directly after the require.js file.
You could add your css files like this with a condition.
I also found an Issue here with a similar problem, but it did not get an answer: https://github.com/magento/magento2/issues/2004