Currently the batch operations are limited to record addition, update and deletion only, is it possible to also add a new method which works like Execute(aSql: utf8string) but is handled by the batch mechanism?

Corresponding forum post: http://synopse.info/forum/viewtopic.php?pid=18976

It was on purpose that direct SQL execution in a batch is NOT implemented in mORMot.
It breaks most interest of the ORM implementation.
Even the "EngineList" method is a PITA.
Then you would be stuck with a single DB provider.

Of course, as we wrote in the doc, sometimes, there is nothing better than a tuned SQL. The ORM can't do anything.
If you want to run some complex SQL statement on the server side, do not use the ORM, but the SOA feature of mORMot.
Write your own SOA service, running directly all the SQL statements on the server side, and call it from the client side.

See this forum thread for discussion.