Please find below a very short answer that agrees with Janus on most points.
You owe a duty to persons *whom it is foreseeable are likely to be harmed by your conduct.
If the only thing I do is replace the relative pronoun with a personal pronoun and adjust the sentences only just enough to make it work, I get this:
You owe a duty to persons. It is foreseeable they are likely to be harmed by your conduct.
The relative pronoun should therefore have been who, not whom, despite what lawmakers write (whom are not always the best of writers).
So what I did was only this:
whom → them/they
change the position of the pronoun to fit a main clause, i.e. before its verb, are
The next step was to choose between they and them, and the obvious choice was they.
It is clear that it is foreseeable needs to be followed by a finite that clause, even though that can be omitted—which is unremarkable, as most conjunctions that can be omitted.
It is also clear from this that it is foreseeable is not parenthetical; the sentence it is foreseeable they are x clearly has the same construction, and it is foreseeable is clearly not parenthetical there, but a superordinate clause governing a subordinate that clause.
The reason why some people write whom in such cases is contamination; they confuse this type of sentence with a different type:
...persons whom I consider to be wrong
...persons whom one knows to be wrong
These are examples of accusatives with infinitives, which indeed require whom; notice the infinitives be:
...persons. I consider them to be wrong.
...persons. One considers them to be wrong.
