Using Service Desk 3.15.1 on Server 7.12.1, I already have a couple of working automation rules that are triggered WHEN Comment added, and also have conditions of "IF Issue matches", and then I have some approving person in a custom field (that came out-of-the-box in SD): CAB in one case, and Change Managers in the other. I would like to do the same for the field "Approvers", but curiously, it is not present in the list of fields for the Issue matches condition. If I enter Advanced mode for that condition and write a JQL statement to "force" it, I see this error:

(transcription: Field: 'Approvers' is not searchable, it is only sortable")
So it would seem that Approvers is a little bit special or something. Is there a way to evaluate its contents for a match with the currentUser() function in a JQL statement? Or will I have to hide Approvers and replicate all its functionality on my own in a custom field?
As an aside, though I don't think it matters here, we specifically are looking to process these responses from "Customer" users, not internal, licensed agents. The idea is that management and up type of employees can issue a statement of approval in a reply email, no logon, no webpage visits.