A simpler workaround is that I would forget about trying to film the actual IR beam. Instead, use a small hidden visible wavelength laser pointer to create a new beam in nearly the same position and direction as where the original beam would be, then film that, using a hand sprayer with a fine mist of water to reveal the beam against a dark background. Then explain in the narration that this is what the beam would look like.

Hopefully this would convey the point without the pain of having to film and deal with the IR issues. I know it won't be as cool as doing the real thing, but if it avoids a lot of trouble...