This lead me to think differently. Because I don't need the old "non redirected" items pages to be online, I can just return DECLINED when  the /<id>/ match, so rewriterule will be used. For files, I just redirect everything to the new page, not file by file. Also, renaming the old repository seems the best option, so we can use the site url just to do redirects.

Thanks to all for the precious suggestions and code examples, very useful.

However, this is no use if you want to redirect document URLs.  I am not sure whether that is what you want to do?