Even in those cases, the quantity of information you need stays constant. Now, you need their time zone. With UTC everywhere, you need their hours of availability.
There is currently some implicit transfer here, in that if they're in an office in Timbuktu, you can figure out their time zone without asking. But you should still coordinate with them, at which point you can find out their hours.... And I'm sure that in a world where everything is UTC, looking up a particular place's customary business hours will be no more difficult than it currently is to look up a particular place's time zone.
There is currently some implicit transfer here, in that if they're in an office in Timbuktu, you can figure out their time zone without asking. But you should still coordinate with them, at which point you can find out their hours.... And I'm sure that in a world where everything is UTC, looking up a particular place's customary business hours will be no more difficult than it currently is to look up a particular place's time zone.