There's no proper documentation on the Maven site descriptor "custom" tag.
It's not mentioned in the main documentation , and is only mentioned in passing in  - which in turn is not referenced from 
Here are the full details from :
custom DOM Custom configuration for use with customized Velocity templates
The DOM reference above appears to be an instance of Xpp3Dom .
Also, the Doxia code sets various context variables (in DefaultSiteRenderer ?).
This should all be documented, along with some simple examples, and referenced as necessary from the Maven site plugin documentation