I tried a quick fix with simple javascript redirects like we do in hibernate.org, but even though the redirect pages are generated, they are not accessible on staging.in.relation.to.

I suspect it's a problem with the Apache server configuration.
Guillaume Smet if one day you're interested, maybe you could have a look? My code, which I checked generates the right pages, is available at https://github.com/yrodiere/in.relation.to/tree/redirects