I don't have a fltplan.com account, so I can't play around with this, but there are a couple of other options. You might try VIX, which is the ICAO code for the SkyStar Vixen and Voyager, or if that doesn't work, try this, from FAA Order 7360.1D:

"ZZZZ – Aircraft type not yet assigned a designator, with a description of the type in Item 18 (Other Information) of the ICAO flight plan or Remarks of a NAS flight plan."

If neither of those are a suitable solution, you could try emailing fltplan.com support at: support@fltplan.com