iRule for syncing iApp over sync-only device group

I think you're out of luck with an iApp, but it should be possible if you configure the objects with tmsh, although I haven't tried this. Create a subdirectory of the sync group's folder and disable device group inheritance. For example, if your /Common is your sync group, put all your common elements here:

sys folder /Common {
    device-group /Common/config-sync-group
}

and create your local-only config in this path:

sys folder /Common/local {
    inherited-devicegroup false
}

I am keen to know how you go with this