We've just been through this argument in the UK, and defeated it thankfully. What was proposed here was indeed a licence to right grab with little effort required to prove you've searched thoroughly. It was sufficient to just declare you never got a reply from the copyright holder.
Even with the requirement for an exhaustive search, the danger is that it's still possible to miss the copyright holder by not looking in the right place. For example a local photographer who sells his work in a store, doesn't publish on the Internet, his work is sold on to another and so on and the trace is lost back to the copyright owner.
For a modern example, someone publishes on Flickr, someone else steals the photo (has happened to me) and it gets copied around with the link to the original copyright owner lost. It then becomes an Orphan work, a company takes ownership and uses it for commercial use, the original copyright owner finds it being used and has no rights.
Orphan works also affect derivatives. Someone takes your photo to use in a derived form, and again it's possible to right grab the copyright because you can claim you can't find the original source. The Internet is full of derivatives (a large proportion illegal, though the US throws around "fair use" as an excuse, which is a concept we Brits don't have in copyright law).
It's far too easy for corporates to put in place enough documentation with ticks in boxes to claim they have done a thorough search, for example by having a fixed list of sources they would traditionally search, which of course misses out anyone who doesn't use those sources.
Don't go down this road
We've just been through this argument in the UK, and defeated it thankfully. What was proposed here was indeed a licence to right grab with little effort required to prove you've searched thoroughly. It was sufficient to just declare you never got a reply from the copyright holder.
Even with the requirement for an exhaustive search, the danger is that it's still possible to miss the copyright holder by not looking in the right place. For example a local photographer who sells his work in a store, doesn't publish on the Internet, his work is sold on to another and so on and the trace is lost back to the copyright owner.
For a modern example, someone publishes on Flickr, someone else steals the photo (has happened to me) and it gets copied around with the link to the original copyright owner lost. It then becomes an Orphan work, a company takes ownership and uses it for commercial use, the original copyright owner finds it being used and has no rights.
Orphan works also affect derivatives. Someone takes your photo to use in a derived form, and again it's possible to right grab the copyright because you can claim you can't find the original source. The Internet is full of derivatives (a large proportion illegal, though the US throws around "fair use" as an excuse, which is a concept we Brits don't have in copyright law).
It's far too easy for corporates to put in place enough documentation with ticks in boxes to claim they have done a thorough search, for example by having a fixed list of sources they would traditionally search, which of course misses out anyone who doesn't use those sources.
Don't do it. Learn from the UK.
http://www.stop43.org.uk/pages/pages/read_more.html
The better option is as proposed by groups in the UK, is to offer true orphan works as a cultural use and open non-profit public domain work.