Requiring all self links be absolute creates difficulties for portable catalogs. The recommendation here is that for portable catalogs there only needs to be a single absolute self link at the root of the catalog. I'd recommend removing the requirement that all self links be absolute and instead require an absolute link exists at the root of the catalog

If you can't provide absolute links, don't provide a self link at all. I don't know what a relative self link really helps with?

Fair enough, I was under the impression that self links are required but after another quick read through the spec that doesn't seem to be the case

Indeed, they are optional, but if you provide them they must be absolute.