From a fellow genealogist point of view rather than a WIkiTree angle, to have Fred Smith married Daphne UNKNOWN shows that you have a bit of information missing, but would be grateful for someone to fill the blank for you. So if someone comes up with a Fred Smith with the same details (birth/marriage/death) who is married to a Daphne Brown, the chances are this is your Fred and Daphne and you now have her surname.
I prefer to use UNKNOWN in Block Capitals on my genealogical software, as this shows up in printed lists of data. If I have an unknown forename and surname, I normly leave the person empty, if I don't know one half of the name, it's "Fred UNKNOWN" or "UNKNOWN Smith".
As others say, a standard is needed, strange as genealogy has been around so long that there isn't!