Seconded the need for localId to be a feasible lookup key. Our apps uses localId to identify the specific macro on a page (among many instances) and is needed for putting the right data in the right macro.
Also question did you consider entity properties to store the snapshots? 32KB JSON is a constraint but simpler to access and work with then registered custom content per app. And that would be consistent with the byline modules that render based on entity properties.