By "current" firmware do you mean the regular firmware or the beta firmware ?
In summer during the day when I want everything off I usually just use the Heating Off action, as this allows me to leave the hot water active.
In the Beta firmware if any zone set point (at the controller) is more than 2C below the measured temperature the heat demand from the HR92's in that zone will be ignored, so that takes care of anyone turning up an HR92 as although the valve will open the boiler won't come on.
If I want to turn on a bathroom radiator alone with the beta firmware I would probably just use my custom action - which is set for 5C permanent. In effect that makes it like heating Off but with the ability to manually override zones.
Maybe the custom option will suit your needs ? Just set the custom action schedule to be 5C at midnight and 2am every day of the week, and apply it to all zones. When you switch to custom all zones will go to 5C, however if you override any zone manually it would go back to 5C at midnight or 2am respectively.
By the way you can also rename the custom action - call it "Summer" for example...
Custom also lets you exclude certain zones so for example you might want to exclude bedrooms which have schedules that only have them on at a low temperature overnight and off during the day - that way if it did get cold in the middle of the night your custom schedule isn't preventing your bedroom radiator coming on but is still preventing the rest of the house coming on.