The indication is that you can use an OpenPanelAction to reference a panel that's not part of the panel.cfg for the aircraft that's assigned to UserOwnship (or whatever aircraft is selected at the time the action fires), in this case, referencing one of the panels defined in missionpanels.cfg.I don't think this is possible currently because script actions basically rely on a true/false kind of result to fire. You could check for certain altitude ranges, and create text for each, but that is not likely what you want. You could make a global mission gauge (defined in gauges\missionpanels.cfg) that displays custom text using lua. Then you could use a panel open action to display your text gauge and it would not require modifying panel.cfg files for different aircraft.
I don't understand how to do this, and would like to. As far as I can tell, OpenPanelAction's PanelID field only looks at the panels defined in a specific aircraft's panel.cfg, serialized from 0 to whatever the highest number window is defined. It doesn't seem to accept the panel "ident" field nor does it serialize the global (mission panels) in order after the panels specific to the aircraft.
It'd be useful to have a way to define global panels like this (especially since Scaleform is deprecated), and I came across the abovementioned topic looking for a way to do just that, but it stops short of actually answering that question. Any help would be appreciated.