I see Wikipedia now has an official mandocello page,
Wikipedia Mandocello
so the rumors are true, they do exist!
However I think the list of 20th Century players of note could be expanded quite a bit:
Norman and Nancy Blake, Peter Ostroushko, John Niemann ( Mando Boys), John Imholz ( MMQ), Joe Weed, Larry Sledge, Dave Richardson (Boys of the Lough - 10 string but still)