The start_session function in lxsession.c reads from /etc/xdg/lxsession/LXDE/autostart and does not provide any way for the user to override this file. Instead, it would be better to check if the file exists in the user's home directory, and if so, to ignore the system-wide file.
Thanks for your bug report. Currently, you can override it by using an autostart file in the home directory. You need to use a newer version of lxsession (git or the next 0.4.9.4 release). I'm closing the bug report for now, feel free to reopen it if it's not fixed.