In my project directory I have svn:externals set to the following:
library svn+ssh://MY_USERNAME@some.server.com/library
where MY_USERNAME is my personal username. It works for me, but if another person checks out my project it will fail since they can't log in with my username. Is it possible to somehow make MY_USERNAME become the username of whoever checked out the project or the username used when running svn update on the library folder?
Maybe try to create another user with read only access?
Another option that you can try is to try Keyword substitution but I have no idea if it will work on properties.
Have you tried relative urls?
../ Relative to the URL of the directory on which the svn:externals property is set
^/ Relative to the root of the repository in which the svn:externals property is versioned
// Relative to the scheme of the URL of the directory on which the svn:externals property is set
/ Relative to the root URL of the server on which the svn:externals property is versioned