I’d like to see this developed as a plugin. It’s a feature a minority of installations would use, and it would be pretty easy to build and maintain this separately. Themes could then opt into supporting it when it is activated.
If you want a quick and dirty approach, this could be implemented as a theme option. Add a theme option for each group, which is just a text field, where journal IDs can be entered in a comma-separated list. This list could then be extracted and used to change the homepage display. No new core features are required.