Hi Sebastian, the mapping toolbox is not implemented for Piwik tags because sending custom data is not a matter of just adding query string parameters to a pixel url.
_paq.push([ 'setCustomVariable','1','VisitorType','Member' ]);
If you have a data layer variable called visitor_type, you could put this code in a JS Code extension and you would send the variable to Custom Variable 1:
_paq.push([ 'setCustomVariable','1','VisitorType', b.visitor_type ]);
In a JS Code extension, "b" is a reference to the data object, so b.visitor_type is your visitor_type variable.
Does that help solve your problem? If you have a lot of mappings to add, then we could put in a request for the mapping code to be implemented in the tag to support this.