I'm clearly missing something, as I don't understand that, BB.
The existing AMC recommends but doesn't demand a logbook signature (i.e. it says "should" not "must").
Why would there need to be an additional AMC statement to deal with the situation where the current AMC recommendation ("should") is not taken up?
The situation is surely in practice anyway covered by the Form 1119x asks for the date of the nominated Revalidation flight, which is in the logbook (along with Instructor name as P1) for the Examiner to see when signing for Reval by Exp.
(The above assumes that MJ or anyone else doesn't find an over-riding requirement for all instructional flights to be signed for every time, which it appears may not exist).
The reason this is interesting to me is that in the last year or so I have trained for 3 Ratings/Certificates and engaged with 3 different training organisations. None of them have had their instructors sign every logbook entry.
(Edit: Actually, it is 4 training organisations, as the lack of logbook signatures hasn't been queried by the examiner who signed my Cert of Reval.)