If I get your idea right, you ask for a way to define surveywide variables without having to create a hidden question inside the survey, right?
Your idea could be mixed with the panel integration feature.
Variablename
Variabledescription
Value external (url parameter name)
Value internal / Defaultvalue
Saved in results (yes/no).
As the "workaround" with the hidden questions works, I remember no feature requests or plugins around static variables.
I recommend to open one and start collecting a few ideas and screenshots from other tools.
Another missing part are static and dynamic lists, which can be applied in different ways.
bugs.limesurvey.org/view.php?id=11688