For this, and for the criminal justice use case, it seems to me that it isn't possible for "work on open source" to receive this kind of formal recognition. Anyone is free to self-certify that they're working on an open-source project headed by, and exclusively contributed to by, themselves.

You'd need to formally recognize open-source projects that the German state approves of, on a case-by-case basis.

And even then you have questions like "If Hans Reiser is sentenced to community service for killing his wife, can he satisfy that by working on reiserfs? How is that different from sentencing him to no punishment?"