You can set the user preferences to store the value in the Value property for the logged in user. This is not accessible to other users. Any JSON can be stored in the property, but it is returned as a string.
You can set the user preferences using the following example POST:
. . /api/v1/User/UserPreferences
Payload: {"Value":"UserPreferencesToStore"}
You can also get the user preferences using the following GET:
. . /api/v1/User/UserPreferences