> ... because you have to select all nodes MANUALLY.
No, you do not. Select the first element manually, then press and hold SHIFT then select the last of the (consecutively) listed elements of the same value. Two clicks to select an arbitrary number of nodes.
> ... which select all nodes with a specific user-defined attribute WITHOUT changing the appearance of the nodes ...
Selecting nodes in the structure view is exactly the same as selecting node by mouse click or "Tools" -> "Select Elements". Of course, selected nodes are highlighted in the editor, but none of their properties are changed by selecting.