Subsites: Show modeladmin in backend on all subsites
By default subsites module disables all custom modeladmin classes in backend when you're not on the Main site (SubsiteID 0).
You can add your ModelAdmins to all subsites by adding this config to your config.yml:
MyCoolModelAdmin: extensions: - SubsiteMenuExtension
If you need more fancy logic on which Subsite your admin should be shown simply create a method called subsiteCMSShowInMenu() which checks if all conditions are given and returns a boolean value.