I would say make a wish, but there is already a feature request in the system for this. Make a wish anyway. I would make a more complete write-up of your use case and get it to your Cisco/Meraki team to add it to the FR for the UI team to review.
The only workaround I could think of for now is to leverage the API to script time-based changes to update the group policies for the clients. So you could have a 12-6 policy, 9-5 policy, and 8-11 policy, which could all be the same or different, no matter, and at the appropriate 3 times of the day, iterate through the clients and assign them to a group policy. But that's an awful lot of API calls if you have a lot of clients. I do not (yet) see the API endpoint for updating GP, only listing them.
Sorry but at least for now, the GP schedule is only on/off once per day and right now no way to schedule multiple discontiguous time windows. Having different on/off hours on the weekend is supported and simple, but again only one on/off window per day.