Occasionally to prune to target (collars, unions etc) would leave very little of the trees biomass, an alternative is
heading cuts preferably back to nodes.
Guy Meilleur aka Treeseer has been an advocate of this practice.
Often the only alternative to this for trees would be removal.
A PDF 1.68mb sitting at this URL to get the ball rolling.
www.treeworld.info/manualuploads/nodepruning.pdf
The Australian Standards AS4373 2007 has made allowance for the similar practice to rejuvenate trees in similar predicaments.
There's follow up work to be done with selecting stronger well positioned shoots to take the role of the new leader.
I'm sure Guy will back this with some supportive documents and evidence.