For the record, I successfully located the bird on Wednesday 30 May, at Mt Talau on Tonga’s second island, Vava’u, mainly thanks to this:
http://www.xeno-canto.org/species/Pachycephala-jacquinoti
... because not only did it let me identify spots to find it, but allowed me to cheat and call it up using a download of its own call on my smartphone. (I know, I know, so shoot me. I had 1 hour left on a tropical island – what would you have done?)
I found it 100m into the trees above the Mt Talau radio tower, after a very obliging taxi driver drove up a track no self-respecting goat would claim, and thanks to my internet cheat I found it within 2 minutes of leaving my somewhat bewildered fellow travellers (2 Fijians and a Tongan, who just accepted it as Western eccentricity,) below the human-induced tree line.
The Tongan whistler has in the past been lumped with the golden whistler, & there is a similar species in Fiji, but unlike the Australian & Fijian golden whistlers, the Tongan whistler has no white bib & just has a full black hangman’s hood. For the record, it was a cracker – a little golden jewel who regarded me and my recorded message with what I can best describe as a disdainful eye.
(This Tongan stamp was the only image I could find of the Tongan whistler on line:
http://www.bird-stamps.org/images/stamps/tonga/1427.jpg)
And then I got to fly back to Nuku’alofa on a 1945 Douglas DC3. A top day was had by me, at least.