Why not Stilt Sandpiper?
I know its not on the HK list yet, but this bird isnt right for Knot/Great Knot/Grey-tailed Tattler, as can also be seen by the range of views.
The relatively plain back with central black shaft streaks, the fairly prominent whitish supercilium, the longish slightly decurved bill [possibly fore-shortened by the photograph angle], the longish yellowish legs and the barred underparts all indicate possible Stilt Sandpiper in non-breeding plumage.
I have quickly looked for a similar plumage bird on the internet and give below link to a pic which shows some of the features [but not the breast barring].
Please post any other photos of this most interesting bird, and let us know where at Mai Po and at what time it was seen.
Mike C
