this can be done with the guionobject
and not just that, by combining guionobject with fxguisnooper you get security cameras and rtt scopes
the stuff needed for rtt cam/scopes would be thishttp://forums.torque3d.org/viewtopic.php?f=26&t=92#p1883
and regarding in model hud, you can read about it in the guionobject threadhttp://forums.torque3d.org/viewtopic.php?f=26&t=92
i assume you would want to display the actual ammo of the weapon in the gui that is being rendered on the gun itself, thats doable.
check out Azazezel`s guionobect branch on githubhttps://github.com/Azaezel/Torque3D/tree/guiOnObject
gl and share how it went.