This may be helpful for anyone else struggling to get complex nested R package dependency installation from the tool shed sorted out. That whole can of worms. While we have setup_r_packages, the developer still has to figure out the right magical incantation and make sure the tarballs are available.

https://wiki.galaxyproject.org/SetUpREnvironment has some notes I've started - contribitions welcome. 

It has a more or less reusable R script to generate tool_dependencies.xml boilerplate, assuming you set the constant libdir to your local git repository path where those tarballs will be downloaded from.

I hope this helps someone!

Could make a tool to do this if enough developers want access to it without the pain of managing yet another R script?