It is not possible to change the tree with GuiXT. What you can do is to use a GuiXT script that leaves the called transaction immediately; e.g. you could put the statements

if not Q[role=administrator]

  Message "E: System administrators only" -statusline

  Enter "/N"

  Leave

endif

into the GuiXT script of the first screen of this transaction in order to restrict it to those users who have the role of "administrator".