Retrieving ROMs from a nix-store on a file-system is awkward because ROM label prefixing destroys labels after the request passes through too many levels of parents.
The prefixing mechanism is used to implement access-control policies, but if ROM requests were made by a content hash, the request would effectively be a request by content capability. In this case no access policy checks can be avoided because the party making the request can be assumed to have some knowledge of the content already.
This could be implementing by using ERIS URIs as ROM labels, and skipping the label prefixing at the parent-child threshold. ROMs would not necessarily need to be store in th ERIS encoding, ROMs could simply be stored in the core image by URI, or likewise on a file-system.
With this in place, programs and libraries could be elf-patched to load by ERIS URI rather than name or file-system path.