Can a multi-valued option be applied to runtime variables for RPE reports in RTC?

Using the Reportable REST API, we have constructed a query that was inserted into the Request URL field of the "Edit Configuration Metadata for Workitems" window of our RPE template.  This query is then used to populate a picklist that the user can select from at runtime.  However, the list is only single-select and our users would like the dropdown to accept multiple selections.  Is there a multi-select option that can be configured in RPE/RTC or is this currently not possible?