One way to tackle this issue is to create a dynamic queried lookup which returns the Project ID. In the lookup the display attribute should be the Project Name and th key should be the Project ID.
This lookup should be mapped in the portlet. This way you can get Project Name to be multi selection type in the filter section.
From what I have observed, the Multi- valued select in the filter section, works only when the attribute is Numeric type.
Regards,
Ashwini