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]