As others have mentioned, you'd have to limit it to those crimes whose statutes have passed.
For crimes with no statutes of limitation (such as murder), I think that you'd have to establish an arbitrary number of years when the crime has gone cold and ceased active investigation.
Even if it was possible for this to "do good", I don't think society would be willing to accept it.
If people *knew* such and such killed their child, many would pursue the vengeance the justice system has washed its hands of.
The only way I could see this working is similar to a confessional, where the person's deeds and anonymity are protected, but the relevant information can be released to the police/victims as needed.
Honestly, there's a lot of programs like this out there already, but you know... it depends a lot on trust.