Index: arch/arch-core-launcher.xml
===================================================================
RCS file: /cvs/core/arch/arch-core-launcher.xml,v
retrieving revision 1.52
diff -u -r1.52 arch-core-launcher.xml
--- arch/arch-core-launcher.xml	8 Jul 2007 10:58:34 -0000	1.52
+++ arch/arch-core-launcher.xml	19 Jul 2007 10:48:19 -0000
@@ -567,6 +567,16 @@
                     </p>
                 </api>
             </li>
+            <li>
+                <api type="export" group="property" name="org.netbeans.core.systemfilesystem.custom" category="friend">
+                    <p>
+                        Contains class name of a class that can serve as provider of the writable layer of the
+                        system file system. The class must be a subclass of 
+                        <code>org.openide.filesystems.FileSystem</code> and have public default constructor.
+                        Morevoer the class denoted by the property must be on the classpath.
+                    </p>
+                </api>
+            </li>
         </ul>
         <p>
             There are also some options passed to the launcher (interpreted
Index: startup/src/org/netbeans/core/startup/layers/SystemFileSystem.java
===================================================================
RCS file: /cvs/core/startup/src/org/netbeans/core/startup/layers/SystemFileSystem.java,v
retrieving revision 1.10
diff -u -r1.10 SystemFileSystem.java
--- startup/src/org/netbeans/core/startup/layers/SystemFileSystem.java	16 Mar 2007 20:52:49 -0000	1.10
+++ startup/src/org/netbeans/core/startup/layers/SystemFileSystem.java	19 Jul 2007 10:48:20 -0000
@@ -279,7 +279,21 @@
             user = l;
         } else {
             // use some replacement
-            user = FileUtil.createMemoryFileSystem ();
+            String customFSClass = System.getProperty("org.netbeans.core.systemfilesystem.custom"); // NOI18N
+            if (customFSClass != null) {
+                try {
+                    Class clazz = Class.forName(customFSClass);
+                    Object instance = clazz.newInstance();
+                    user = (FileSystem)instance;
+                } catch (Exception x) {
+                    ModuleLayeredFileSystem.err.log(
+                        Level.WARNING,
+                        "Custom system file system writable layer init failed ", x); // NOI18N
+                    user = FileUtil.createMemoryFileSystem ();
+                }
+            } else {
+                user = FileUtil.createMemoryFileSystem ();
+            }
         }
 
         if (homeDir == null) {