[Justin]

Yes, some of the presets implement this, but here's a preset which, if you put it last on a track, will only make the track FX used when the track has video items:

// makes track FX only used where track has video items
input = input_track(0);
input > input_next_item(0) ? input=0;

input_info(input,project_w,project_h);
gfx_blit(input);

If you're using the latest +dev builds, this can be optimized slightly:

// makes track FX only used where track has video items
input = input_track(0);
input <= input_next_item(0) ? (
  input_info(input,project_w,project_h);
  gfx_blit(input);
);

Also, you can of course put video FX on video items, or on MIDI items if you want them to operate only for that time period (they can still read video from tracks upstream etc).

Caveat: this doesn't handle being the only FX on a track properly (will figure out a workaround for that -- not that it's useful as the only FX on a track).