Bruce Nielsen provided this answer at at a no longer reachable thread on the old ArcGIS Discussion Forum and it seems to have been accepted there.
Since a multivalue input parameter is a text string delimited by semicolons, I would think the output parameter would be the same. In theory, you should be able to create the appropriate output with the following change to the example code:
arcpy.SetParameter(3, ';'.join(listOfClippedOutputLayers))