I only discovered this myself by chance yesterday…
Open the mixer and press the Inserts button to expand the view. Scroll to the Reverb channel and press the ‘e’ button above REVerence in the Reverb channel to show the plugin.
Press the Browse button to choose a different reverb. I quite like the Bright Viennese Hall, which suits orchestral and piano music more than the LA Studio default.
In the REVerence window press the diamond button in the top strip and in the menu select ‘Save as Default Preset’.
Now the next time you create a new score or apply the default playback template then this will be chosen as the default reverb.
Incidentally, the problems with not being able to apply FX to non-HALion plugins will be fixed in the next update.