- Added user defined python dir to Python's sys.path:

Had to add a function call to creator.c, explained in a note there (look
    for pythondir in the file)

source/blender/python/BPY_extern.h

@@ -47,6 +47,7 @@ struct _object;  // forward declaration for PyObject !
 
 void BPY_start_python(void);
 void BPY_end_python(void);
+void BPY_syspath_append_pythondir(void);
 int BPY_Err_getLinenumber(void);
 const char *BPY_Err_getFilename(void);
 /* void BPY_Err_Handle(struct Text *text); */

source/blender/python/BPY_interface.c

@@ -178,6 +178,9 @@ void init_syspath(void)
 
 	if (U.pythondir) { /* XXX not working, U.pythondir is NULL here ?!?*/
 					/* maybe it wasn't defined yet at this point in start-up ...*/
+					/* Update: definitely that is the reason. We need to start python
+					 * after U.pythondir is defined (better after the other U.xxxx are
+					 * too. */
 		p = Py_BuildValue("s", U.pythondir);
 		syspath_append(p);  /* append to module search path */
 	}
@@ -192,6 +195,18 @@ void init_syspath(void)
 	}
 }
 
+/*****************************************************************************/
+/* Description: This function adds the user defined folder for Python        */
+/*              scripts to sys.path.  This is done in init_syspath, too, but */
+/*              when Blender's main() runs BPY_start_python(), U.pythondir   */
+/*              isn't set yet, so we provide this function to be executed    */
+/*              after U.pythondir is defined.                                */
+/*****************************************************************************/
+void BPY_syspath_append_pythondir(void)
+{
+	syspath_append(Py_BuildValue("s", U.pythondir));
+}
+
 /*****************************************************************************/
 /* Description: This function will return the linenumber on which an error   */
 /*              has occurred in the Python script.                           */

source/creator/creator.c

@@ -388,6 +388,15 @@ int main(int argc, char **argv)
 		sound_init_audio();
 
 		BIF_init();
+
+		/**
+		 * NOTE: the U.pythondir string is NULL until BIF_init() is executed,
+		 * so we provide the BPY_ function below to append the user defined
+		 * pythondir to Python's sys.path at this point.  Simply putting
+		 * BIF_init() before BPY_start_python() crashes Blender at startup.
+		 */
+
+		BPY_syspath_append_pythondir();
 	}
 	else {
 		BPY_start_python();