Use relative externals, looks like mr.developer supports it

sources.cfg

 [sources]
-AccessControl = svn svn://svn.zope.org/repos/main/AccessControl/trunk
-Acquisition = svn svn://svn.zope.org/repos/main/Acquisition/trunk
-DateTime = svn svn://svn.zope.org/repos/main/DateTime/trunk
-DocumentTemplate = svn svn://svn.zope.org/repos/main/DocumentTemplate/trunk
-ExtensionClass = svn svn://svn.zope.org/repos/main/ExtensionClass/trunk
-initgroups = svn svn://svn.zope.org/repos/main/initgroups/trunk
-Missing = svn svn://svn.zope.org/repos/main/Missing/trunk
-MultiMapping = svn svn://svn.zope.org/repos/main/MultiMapping/trunk
-nt_svcutils = svn svn://svn.zope.org/repos/main/nt_svcutils/trunk
-Persistence = svn svn://svn.zope.org/repos/main/Persistence/trunk
-Products.ExternalMethod = svn svn://svn.zope.org/repos/main/Products.ExternalMethod/trunk
-Products.MIMETools = svn svn://svn.zope.org/repos/main/Products.MIMETools/trunk
-Products.PythonScripts = svn svn://svn.zope.org/repos/main/Products.PythonScripts/trunk
-Products.ZCTextIndex = svn svn://svn.zope.org/repos/main/Products.ZCTextIndex/trunk
-Record = svn svn://svn.zope.org/repos/main/Record/trunk
-tempstorage = svn svn://svn.zope.org/repos/main/tempstorage/trunk
-zExceptions = svn svn://svn.zope.org/repos/main/zExceptions/trunk
-ZopeUndo = svn svn://svn.zope.org/repos/main/ZopeUndo/trunk
+AccessControl = svn ^/AccessControl/trunk
+Acquisition = svn ^/Acquisition/trunk
+DateTime = svn ^/DateTime/trunk
+DocumentTemplate = svn ^/DocumentTemplate/trunk
+ExtensionClass = svn ^/ExtensionClass/trunk
+initgroups = svn ^/initgroups/trunk
+Missing = svn ^/Missing/trunk
+MultiMapping = svn ^/MultiMapping/trunk
+nt_svcutils = svn ^/nt_svcutils/trunk
+Persistence = svn ^/Persistence/trunk
+Products.ExternalMethod = svn ^/Products.ExternalMethod/trunk
+Products.MIMETools = svn ^/Products.MIMETools/trunk
+Products.PythonScripts = svn ^/Products.PythonScripts/trunk
+Products.ZCTextIndex = svn ^/Products.ZCTextIndex/trunk
+Record = svn ^/Record/trunk
+tempstorage = svn ^/tempstorage/trunk
+zExceptions = svn ^/zExceptions/trunk
+ZopeUndo = svn ^/ZopeUndo/trunk