Together with my colleague Marek Kuzminski, we have slightly modified Brian's scheduled maintenance mode MP (Scheduling Groups of Objects for Maintenance Mode) to add following features:
I wasn't able to re-use that MP to do different schedules just by overrides. So the design is still rather 'quick', and limits your number of schedules to 10 (number of rules). To use overrides for different schedules and computer groups bound to the same rule, the MP would have to be redesigned.
Thx to Brian for posting his great MP, and Marek for his help.