My NFeed field has "admin" as a choice and it is selectable in the UI. Via, Rest, the below Json doesn't seem to work, although it doesn't error either ( 10202 is my NFeed User field )
"update": {},
"fields": {
...
"customfield_10202": [
{
"value": "<my email address>"
}
]
}
This actually clears the NFeed field. Using the user name and key instead of the email address have the same result.