To run a Xwindow application, you have to have a X window manager running - if you don't it's not going to happen. You need to start reading the man pages on X11, xdm, Xsun (don't know what HP and IRIX use in that case).
You stated you "already have a way to bypass the login". If you are simply moving dtlogin startup then you won't have a window manager running so you can't start any X applications and have them show up.
Also take a look at some of the info on the net.
X Windows (SUN)
Someone else trying the 'same' thing