What a curious question (may you all have peace!).
Excluding this year (due to recovery from surgery), I have attended the Memorial without break since 1980, including since being asked to leave (DF'd) in 1997. Not only have I never been asked (or even had it intimated to me) to leave before the prayer, I think MY doing so (leaving on my own) would actually cause a problem.
I mean, I currently actually get a few sly "dark" stares when folks notice I'm not only not bowing my head... but not even praying WITH them (Seriously? I mean, "Jehovah" is not MY God, so why in the WORLD?). Instead, I simply do what they do, for instance, during the Pledge of Allegiance: stand quietly and somewhat respectfully. Not so much because I actually "respect" what THEY'RE doing/saying... but because I don't want to call [additional] unnecessary attention to myself. What would be the purpose of that? It IS their meeting, after all, so they're perfectly entitled to DO what THEY wish in THEIR meeting. I am merely a guest.
Even so, should it ever come up anywhere in the U.S. where I AM attending (most likely, California)... I would simply remind them... politely and quietly... of the "freedom of religion" clause in the U.S. (and California or other state) Constitutions... and tell them that I am NOT above filing a civil suit but really would "prefer NOT to go there."
Again, a very curious question. What's behind it, dear StandFirm (peace to you!).
Peace to you all!
A slave of Christ,