Currently, any custom fields created in the Designer tool cannot be passed as parameters to a Quick Launch item.  Custom extension fields can be passed as parameters to the command line for an External Command Task, which is used in a planned workflows, but there is definite value for the Quick Launch functionality to leverage data from these custom fields as well.
This becomes especially important when using workflow for Service Requests, and analysts may need to trigger Quick Launch calls to Third Party software applications based on data captured in the Service Request (ie Procurement, Employee Onboarding, etc).