Jonathan wrote:
So I am considering using a Chinese technique from about the same
historical era of using giant kites to lift Thames water into place
above the moat and pour it in from there.

You will probably need a permit to abstract water for moat-filling.

Does anyone have any experience with this particular technique (which, I
understand, will require considerable resources and manpower to
implement), or indeed defensive Angle-Saxon moat building in general?

I would suggest you compromise on authenticity a little and get one of
those self-powered whirly-kites used to drop water on forest fires.

Owain