I'm blogging this because the Red Bean book is confusing, various different syntaxes have worked in different svn versions, and it took me a few minutes to figure out what would work.
I wanted to centralize some XQuery modules that will be shared between different projects in Endings (and elsewhere potentially). This is how I ended up doing it (example from Mariage):
[In the Mariage repo, "static" folder:] [Create a local folder in the repo for all externals:] svn mkdir externals cd externals [Specify a subfolder name which will be external, along with the remote path to the source repo:] svn propset svn:externals "xquery https://revision.hcmc.uvic.ca/svn/hcmc/xquery" . [svn up and svn commit]