Binding to an attached object's signal from outside is virtually impossible. It results in many weird errors on multiple levels.
See attached (whoosh) project for attempts to handle Keys.onReturnPressed signal of a text field, given that text field as a variable/property value.
Here are chunks of that code:
Note that it is possible to assign and pass around attached object by itself, so this is definitely not a general issue of type system; only Connections QML Type seem to be flawed. In the following case we can observe one object being printed twice to a console: