You mind if I ask why you need the artifact's repo URL? Seems like the local artifacts and "resolution graph" ought to be enough info.

An OSGi repository index file contains information about a set of bundles. The resolution graph allows us to define the set of bundles, and we can then introspect the bundles to gather relevant dependency information.

The problem comes with the definition of the osgi.content namespace. Every resource in an OSGi repository has to have a "content capability" which contains the SHA-256 digest of the resource, and the location that it can be downloaded from. Without this information the index is more or less useless to external users. Specifically provisioning systems like Karaf need to know where the bundle can be downloaded from in order to install it "on demand" to satisfy a feature or its dependencies.