Meanwhile I have found why subversion auto-props does not work with EA. EA uses its own configuration directory with subversion, and therefore its own set of rules for auto-props.
During the creation of the subversion settings, EA calls e.g.
<your path>\svn add --auto-props --config-dir "C:\Program Files\Sparx Systems\EA\Config\SVN" "ea9\9FF6F137BE13.xml"
<your path>\svn commit -m "EA Package addition: 19.01.2012 14:55:45 " "ea9\9FF6F137BE13.xml"
There, you'll notice the option --config-dir, where EA uses its own configuration file config, which contains
### Enterprise Architect SVN configuration file.
### ============================================================================
### This file instructs SVN to set the svn:needs-lock property on all xml files
### when they are added or imported.
[auto-props]
*.xml = svn:needs-lock=NL
*.XML = svn:needs-lock=NL
*.eab = svn:needs-lock=NL
*.EAB = svn:needs-lock=NL
The point is: The user may have its own configuration file (as I have), but that configuration is overridden by EA. Consequently, you have to transfer your settings to the configuration file used by EA. E.g., you have to append lines describing your own auto-props.
I'd suggest to EA people to re-consider this behaviour, or at least give a notice to users (in dialog box, or in online help, or FAQ, ...).