We should look at adding support for the extras directory used in publican for example code in programlistings (see http://jfearn.fedorapeople.org/en-US/Publican/3.0/html/Users_Guide/chap-Users_Guide-Creating_a_document.html#sect-Users_Guide-Adding_code_samples).
Somewhat related to BZ # 858764 although it is primarily referring to the content that ends up in files/ rather than extras/ (the difference is that content that ends up in extras/ makes it through the XmlClean process and thus can be xi:included with parse="text" set).
Hehe, yeah I was gonna add it to that RFE, but upon thinking on it, it has enough variations to have it's on RFE. It is also a lower priority then that RFE since this you can workaround it without having to do something outside of the PressGang build process.