Passing Complex Expressions via Data Flow Parameters

Following the patterns set by "Entering the JSON structure manually" and "Assign parameter values from a pipeline", I'm trying to combine the two without success.

If I create a parameter, and attempt to pass in the value @(field1=0), the expression editor shows successful evaluation, but the parameter tab gives an error: Invalid type, expected string and provided complex.

However, complex is not a valid type to select.  Any thoughts on a better approach?

The base data flow will be responsible for performing the "standard" mapping activities.  However, want to allow the option to perform a custom data mapping that doesn't require an update to the base data flow.  I'm looking for extensibility via config to support the one-offs.