Since feature 1793977 was implemented, visual editing of RELS-EXT is now possible. The treatment of RDF prefixes/namespaces is still fuzzy in this editor.
This feature is a placeholder for as-yet-unspecified changes to make the editing/entering of URIs vs. "short" names for URIs more clear. Should there be a set of client-side stored prefixes? Default prefixes? How about display -- should full URIs be shown differently than "aliased" URIs? This all needs to be specified.