Haven't had anyone saying they've seen it, so far.

: Does seem like it would a lead-in to the charge animations or something like it.

Possibly you meant "would be". Yes, i did wonder if it might be analogous to the arm waving an Elite does when starting to rage; but because he's friendly, he doesn't actually attack.

: I guess maybe it got cut and alongside the weird scripting stuff with all the
: 'allied' Flood it might have been left in the tags only for them? Cool find!

Notice also that there's no vocal with the animation. But there clearly should be one. Maybe it was cut, on the expectation that it was never going to get used after all? Or perhaps they never even got around to adding one in?

: Are we certain it can't be done for any other Flood?

We can't be certain of that, short of examining the computer code or hearing from the programmers. But I've killed a gazillion Flood over the years, without seeing it.