Survey Trigger Conditions - User field

[Vernon1]

In the Survey Trigger Conditions form, one of the required fields is the "User field". It is used to determine who to send the survey. Most of the time this would be the caller.
In my circumstance, the table for the survey is the Case table [sn_customerservice_case]. The client has added some custom fields to the table. On the case form they have a custom field for the name and another for the email. These are string fields, not lookup fields. The client has a public facing portal where they enter the name and email information.
In the survey trigger condition, I want to use one of these two fields to send the survey, but they are not presented in the element tree listing for the "User field". In fact, I noticed that no custom fields are listed. It would appear that this field only accepts references for fields of type sys_user.
Is there an OOB way to trigger this otherwise?
Does this mean that we won’t be able to send a survey request to the public users?

[spike]

I've just read that you can change the dictionary entry for this field so that it will accept other field types. Specifically it was talking about the contact table I think. Not sure this helps as I want to be able to put an email in here of someone that's not in the system..  YMMV..