Ah yeah, user settings. It was pretty early.
So just run 'security/user/setting/create' instead, it's pretty much the same but you'll need to pass a user id in the $properties array:
$properties = array(
'fk' => 6, // user id
'key' => 'my.setting',
'name' => 'My Setting',
'description' => 'Setting description...',
'xtype' => 'textfield',
'namespace' => 'namespace',
'value' => 'a default value',
);
User settings
are stored in the user_settings table. But I wouldn't think the code would be much different run from a front-end snippet instead of a back-end processor?