Mystery Flowerpecker
Have we considered the possibility of Buff-bellied Flowerpecker ?
The belly here is very buff-coloured and Tai Po Kau is a good site for Buff-bellied, as well as for Scarlet-backed Flowerpecker.
I have little experience of Plain Flowerpecker, and I'm not qualified to add anything to what is written in "The Birds of Hong Kong and South China". (I'm assuming you have the book.)
According to Robson's "Birds of South-East Asia" PLAIN has "Greenish-olive upperside, pale olive-greyish underside, cream-coloured belly centre and sometimes throat..."
Do you have any other shots of this bird ?
John