I'm trying to use an internal Artifactory server to distribute plugins. As our environment is disconnected to the internet, I've used a connected computer to download all plugins and dependencies and then moved all files under [user]\.ivy2\cache to our network, and used the Deploy Artifacts Bundle feature of Artifactory to deploy them all at once. I've deployed them to a special Artifactory repository that i've set as having Ivy structure.
This is the relevant part from BuildConfig.groovy
This RepositoriesConfigurer object is what you are working with in the repositories block. Check out the other code in there and you should be able to deduce how to create your own resolver and give it to the resover() method.