I've been scouring the internet for flat fingerings for high G, but could never find anything satisfactory. It's a real bane, especially for band music because it's often in the flat keys: G's would have to be tuned rather flat, and frankly, that's not possible by embouchure alone, nor will any headjoint and cork position compromise ever work, especially on the piccolo. (You could re-scale the flute, but that's not quite a viable option...)
One fine night, before a performance, I chanced across an excellent alternate fingering:
X O X | O O X | O
O
and the slightly flatter variant (usually a tad too flat):
X O X | O O X | Eb
O
It is in fact the trill fingering for the F#-G trill, so it isnt novel or anything.
These fingerings have an added bonus on the piccolo because they seem to give a much more consistent response, especially when navigating leaps. (I actually only had one octave leap to high G for that particular performance, but I think I spent more time practising that leap than practising the entire repertoire!) On the flute it would solve the problem of underpitching when slurring from the D below that. Timbre might be a minor issue, though.
Hope this helps.
How to Flatten High G
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How to Flatten High G
The flute family: probing the lower limit of human hearing and the upper limit of human tolerance.