This comes a bit late, but seeing that nobody has replied, I shall.
Always consult an experienced flautist when writing weird stuff. (Well, normal stuff, too, because what may seem normal to you may be incredibly awkward on flute.) In particular, the instrumentalist you have in mind. (Just because an extended technique exists doesn't mean everyone can play it.) Don't expect your average flautist - not even professionals - to know much.
On a normal flute (Böhm-Lot system), the glissandi ranges are as follows:
D1~Bb1
D2~F3
The lower limits of each range can be pushed slightly using embouchure changes, but tone deteriorates rapidly.
The upper limit of the latter range can be extended a little, say to F#3 or G#3, but it's extremely tricky.
The subranges F1~G1 and F2~G2 have very poor response; do not write anything beyond mezzo-piano.
The range Bb2~D3 is not commonly known, and transitioning between the three subranges of the latter range is not smooth at slow tempi.
The subrange A1~Bb1 And A2~Bb2 is not smooth, but it is viable as an illusion when approached from a lower note.
Some special flute alterations can expand these ranges, but don't expect to traverse between them.
Glissandi can be executed on the flute headjoint sans body; consult your local flautist for the range. Do note that removing and reassembling the headjoint is a major technical disruption.
Hope this helps.
KZ
