It turns out that there is a small difference between the default configuration for Vimoutliner and the way I set up my Vim environment. By convention extension scripts for Vim are stored in a ‘plugin’ directory under your Vim home (which is ~/.vim by default) and that is where, amongst others, I had place vo_checkbox.vim and vo_hoist.vim. But for reasons known only to itself I have just discovered that Vimoutliner looks for extension scripts in a directory called ‘plugins’ (notice the plural). So a quick directory add and a couple of svn move‘s later and I was back in business.