Maintainer of `react-querybuilder` here. Couple of things:
Firstly, both RQB and RAQB facilitate the `WHERE` clause of a query, not the `SELECT`, `FROM`, or other clauses that would control aggregation.
Secondly, I would strongly discourage you from passing SQL from the client to the server, even if you parameterize it. For RQB, we recommend passing the native query object as JSON to your API and using our provided export method on the server to transform it into SQL (or whatever format you need).
Once in a blue moon I get requests for aggregation support in RQB, so to address your last question: yes, I think there would be some demand for something like that.