Or essentially allow you to flash text or images on screen for arbitrarily short lengths of time?
From a quick internet search, seems there are some apps out there.
I’m intrigued by your use case for this, do expand!
I’m also now reminded of early Windows scrolling screensavers where they let you set your own message on the screen 
I’ve done the quick Internet search but don’t see anything that does exactly what I suggest… they have a bunch of other random stuff instead (like audio messages, or else they’re for Android only, etc.)
Honestly I was intrigued by the “Mommy and I are one” message outlined in this New Yorker article and wanted to test it out on myself. 
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Found an app that works: Psychopy
It’s free and used for subliminal priming research in psychology. I found that I had to make sure to run the experiment, once I had it setup, in NON-full screen mode the first time (this is an option in Settings → Screen) so that I could make to enable some relevant screen recording permission that it asks for.
I read the article. Interesting. After reading the article it reinforced by suspicion that too many people may analyze and focus on themselves too much, or have themselves analyzed by others, to the point of psychological and emotional unhealth.
I find it fascinating (perhaps sad) that so many “celebrities” are in therapy. I don’t wish to be misunderstood, certainly there is a need and place for therapy and counseling, but I sometimes wonder if too much reliance on psychoanalysis and the like is counter-productive. I don’t know, it is just a hunch.
I do know this, I would never go to a therapist without first knowing his or her presuppositions regarding the nature of the mind, its connection in relationship to the brain, presuppositions regarding the foundations of ethics and moral behavior (which shape how we ought to behave toward ourselves and others) and the essence of human nature. Presuppositions related to these issues and more shape any advice that is given. If the presuppositions are wrong, much of the advice will be wrong. As someone once said, “We may think wrongly upon right principles, but forever do so upon wrong ones, for how can the stream flow pure when the source of truth is polluted?”
As to subliminal messages, I just play my records backwards. 
Now that I think about it, I think my wife has my computer flashing a subliminal “Honey Do List.” 
I am reminded of the Farrah Fawcett poster on my college roommate’s wall with the word sex adeptly “hidden” in her flowing hair as well as a program intending to warn us about the dangers of subliminal messaging in TV commercials (can you spot the skull and crossbones in the beer foam?). I also recall being inspired to test my own hand at hiding messages in photos, such things as tennis shoes where the laces are arranged with “sex”.
Ah, the joy of the late 70’s.
—
JJW
As a teenager, I dismantled and rearranged the tape in some cassettes, to try to play them backward.
When I listened without any assumptions about what I’d hear, I didn’t really hear anything. When I listened for a nonsense phrase I had come up with in advance, I heard things very much like it. 
For what it’s worth, I also remember reading the original of this article, here cited by the American Psychological Association. It can still be fun to mess around with images and recordings, of course.
Ahah – here’s the full text, presumably with no secret messages hidden in the PDF.
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My search was for “subliminal message apps” and found a number of apps and review articles, but I did not evaluate.
I don’t know what answer I was expecting but that isn’t it