Our requirements were using the normal download link file. So we have to intercept the flow. I first think about a attribute in item mets, but I read about it and seems to be a closed specification and I dont want to break it.
There are three behaviors.
1. Dspace works as usual.
2. If the file is restricted it ask to login and if even so the user did not have permissions login shows you the request-copy.
3. It shows the request-copy directly without login.