I've been on the phone to Pembrokeshire CC and Cadw, with mixed responses.
The footpaths specialist at the CC was really helpful, and he's going to take the matter up with the Mathry Parish Council. Apparently they're already doing a survey of public rights of way, working with the parish councils, and prioritising paths that are in or near the national park. The path to Tre Wallter Llwyd (pp 6313) falls relatively well into this area, because it's off a bridleway that leads into the national park (pp6312). The fact that there's a Scheduled Ancient Monument at the end of the path helps as well. He's going to get back to me with an update (I wish our local CC was this enthusiastic about footpaths).
When I rang Cadw I got a relatively subdued response. The guy on the phone didn't seem to have any opinion on the matter, at least not one he was going to share with me. He took the minimum amount of details from me, and said he'd call back if and when anything happened. Perhaps I'm being unfair, but I'd have hoped to have had more feedback from them.
If either of these lines of enquiry goes cold I'll put pen to paper, but so long as I make progress over the phone, I'll stick with that.
Kammer x