What about log_search, which is intended for permanent storage of data; could your extension generate a log event and then use that log_id as the ls_log_id? https://www.mediawiki.org/wiki/Manual:Log_search_table

Maybe there could be a new type of log_type/log_action introduced, e.g. storedata/myextension, if your extension is called MyExtension. Each extension that stores data in this way would have its own log_action, and would generate one or more log events as needed to reserve a log_id(s) for data storage.