Following signal handler works:

property real __private_property
on__Private_propertyChanged: console.log("__private_property", __private_property)

But if I create such signal handler with Connections element in QML component where the property hasn't been defined within the same component

Connections {
    target: item
    on__Private_propertyChanged: console.log("__private_property", __private_property)
}

the component loading fails in error

Cannot assign to non-existent property "on__Private_propertyChanged"

If both the property and Connections element have been defined within the same QML document there is no error. If property uses normal camelCase there is no error.