Per user values?
What about adding another checkbox to custom field properties:
If set, values for such custom field should be stored in separate table, e.g.,
custom_user_values. This table should contain an additional field:
In other words, this checkbox will make it possible for each user to set his/her own value for the custom field. This can be used for:
The problem is how to show such values (all? some aggregate result? should depend on type?).