I really had thought of getting something apparently cleaner , like the syntax you are proposing. At the end I decided it'd be better to preserve an 'argument' field to clearly indicate that it is a function-like definition and we need to pass an argument to it , something more like a spec:
(sig 'sum^mul) => (arg)
Besides being a good visual indicator, it also plays a role inside the macro definition , this is the variable containing the value to be matched against the different patterns internally, so in this case, such a variable is actually needed too.
Pattern-matching operates over 'data structures' , that is why we need to pass lists (or another structure) to the function. I think there is no need to increase complexity on this, or probably creates confusion about it.
