Hi,
we tried to use the JavaDoc Link Option earlier today. After some troubles to get it to work, we found out that the generated javadoc.options file was incorrect - it misses quotes between the option values (external link url and local package list)

instead of
-linkoffline 'http://java.sun.com/javase/6/docs/api/ /usr/lib/jvm/java-6-sun/docs/api'

it needs to be
-linkoffline 'http://java.sun.com/javase/6/docs/api/' '/usr/lib/jvm/java-6-sun/docs/api'

Our solution to this is a small patch to JavaDocOfflineLink.java that adds those to the toString method.

We've tested it here and now it works

An example gradle snippet:

apply plugin: 'java'

javadoc {
options.linksOffline 'http://java.sun.com/javase/6/docs/api/', '/usr/lib/jvm/java-6-sun/docs/api'
}

We would be happy to supply any more information etc to help on this issue