I don't think this is possible, since the workflow scripts are per project, meaning you are already in the project.
However you could set up a separate link to Quality Center using the same format QC produces when you right click an entity and select Copy URL or Send Email. The links in those cases include the QC server name, QC domain, QC project, module, and specific entity. One of those URLs will take the user to a QC logon page where the QC domain and project are already set, and they fill in their username and password. Put that URL on your own web page and have users access QC that way rather than going to the QC login page themselves.