I find it hard to believe that endnote mentor does not know what a working paper is. I am workin gin sociology and a working paper is usually an interrum research report or a paper published by a faculty or research group of a university, for example. Commonly the working paper is part of a series of working papers, usually number from 1 onwards. As the author of the query noted, Mendeley offers working paper reference formatter. So, why not endnote?
Regards,
Allan.