First off, that has not been proven about pi, but it is heavily theorized. Second, you need the substring index, which, for a large file is guaranteed to exceed the original's file size to begin with. Transmitting that substring index is likely equivalent to compressing the file.
Courts aren't robots and people are judged by other people. Nobody will go to jail for holding a file with all possible files in it. But using that file with a specific substring index to extract something illicit, with intent, will almost certainly not make the courts happy. IANAL.
>But using that file with a specific substring index to extract something illicit, [...] //
The question at hand though, is if you don't extract the illicit thing [from the Blockchain] is possession of the Blockchain enough.
FWIW I think possession has to be the measure used for child pornography and similar content, at present. But what is meant precisely by possession may need some interpretation (does one "possess" a file if one has the magnet link?) and may evolve in the future if such Blockchain attacks become of interest to the legislature.
Courts aren't robots and people are judged by other people. Nobody will go to jail for holding a file with all possible files in it. But using that file with a specific substring index to extract something illicit, with intent, will almost certainly not make the courts happy. IANAL.