The reason that a FI/CRI is asked to sign the logbook on a Dual flight is simply to assist the examiner by providing a verifiable endorsement that a particular flight met the requirement for a dual flight as described in JAR-FCL 1.245(c)(ii)(C). It is not a legal requirement, nor is it even a JAA requirement that the log book be signed by the instructor. Prior to the introduction of JAR-FCL in 1999, Examiners asked the CAA how they could verify that a person in a log book was a FI. The CAA agreed that if the FI signed the entry with a licence number the examiner could cross check it with the CAA if necessary. That is not to say this is the only method of checking, but it has proved to be a simple method for the past 12 years.
If a pilot produced evidence of such a flight to the CAA they would have no alternative but to revalidate the rating by experience if all other criteria were met, even if there was no signature.