To add subliminal subtitles into a video

Can you elaborate on what you mean by "wouldn't be explicitly seen, but would be 'there'"?

Two ways I can think to accomplish something like this:

1. display each title for a very brief period, perhaps just one or two frames
2. use an effect to blend the title into the image, something like the frame grab below

The frame is the shortest unit of time for video. The "frame rate" refers to the number of frame per second [fps]. Though there are retail video cameras that shoot thousands of frames per second--and one at MIT that can do a trillion fps--my understanding is that few consumer devices can play video faster than 60 fps. (Content from those super-high-speed cameras is typically used for ultra-slow-motion playback.)

Premiere's encoder caps out at 60 fps.

In the Timeline, you can trim any video clip down to a single frame. If your sequence is set to 60fps, the shortest duration for a clip is one-sixtieth of a second. That's long enough for the image to register in the brain.

In don't know whether you might be able to achieve the desired results by playing a clip at high-speed with an analog device like a VCR, but I'm pretty that approach would not work on a digital device like a computer or a DVD player because they achieve high-speed playback by skipping frames.

I suspect that the solution is going to require special hardware like a projector that can play faster than normal. (btw, "normal" for film is 24 fps.)