Hi!
Data living in an RDBMS engine distinct from Virtuoso is handled via the engines Virtual Database module i.e., you can build powerful RDF Views over ODBC- or JDBC- accessible data using Virtuoso. These view also have the option of being materialized etc..
Yes, but the way the data are stored now is JSON blob within a text field in MySQL. I do not see how RDF View over ODBC would help it any - of course Virtuoso would be able to fetch JSON text for a single item, but then what? We'd need to run queries across millions of items, fetching and parsing JSON for every one of them every time is unfeasible. Not to mention this JSON is not an accurate representation of the RDF data model. So I don't think it is worth spending time in this direction... I just don't see how any query engine could work with that storage.