> this feature is algorithmically impossible
> You're just looking at one specific video, not the general problem.
> is not generally possible.
As a fellow multimedia dev, man, who cares? Sometimes we forget that software ought to be useful, not hypothetical ideals of truth. Just implement the feature for those codecs that support it and which probably are in the 98% percentile of what users actually use, regardless of the damned "general case".
Or accept and announce shamelessly that you don't have either the knowledge or the development resources to tackle such a complex feature. But excuses about not being possible for absolutely every possible codec in a completely generic way is just denying that the world is just a chaotic and dirty place where things are not ideal nor perfect. Just give your users a real-world solution (or rejection).
I've long since come to the understanding that if I want software that are useful, I must go and pay for it. Commercial software exist to make money, and to make money they must be useful.
If I want software that are hypothetical ideals of truth, I go and shitpost at the nearest neckbeard communion. Free-beer software exist to satisfy a man's desire to display his voluminous beard, everything else is tertiary.
Even if it were free, wouldn't you want to do something nice (like give them money) for the person(s) who made the useful thing for you?