I don't believe you need to actually store or track invalid state, just check that each custom_json is valid when processed. If the custom_json creates an invalid state (for example, presents a non-unique tag), the custom_json is invalid and is ignored. This is the usual method for doing any sort of embedded consensus, which is effectively what this is.

In fact, now that I think about it, non-unique might be considered as valid, since referencing an existing id could be an edit, if the rules mirror those for comment ops. In that case, there would be no invalid (non-unique) ids.