I suspect that the cops wouldn’t like the chain public, though.
An attacker altering the ledger would still require compromising an unreasonably large number of independent groups at once, and even then the rest would be able to clearly see that some unusual and suspicious event occurred.
By limiting membership a bunch of problems simply vanish, like long-clearing times, wasting hardware on mining, vulnerability to foreign botnets, etc.
[0] A blockchain is distinguished by its core requirement, from which a cascade complexity flows: Uncontrolled node membership. Don't be fooled by people pitching "private blockchain", its a contradiction in terms designed to rehabilitate hype, like "multi-sample Theranos test" or a bicycle as "Segway passively stabilized inline wheel model."
if it is mandated that every photo or video taken for the possible use in evidence is notarized at the time of acquisition, any fabrication would necessitate total premeditation. that is, the fabricators would need to know ahead of time what they were pursuing and what evidence they would need. this seems like a very costly barrier.
for example, altering security footage would require some fantastical elements: a real-time system of ingesting real footage and altering it in real-time to slip it into the notarization pipeline within the error margins.
requiring that any equipment that produces acceptable evidence stream commitment hashes in real-time to public append-only repositories would be an enormous step forward.