LOAD" is not a Virtuoso SQL command, it's an ISQL one. The ISQL checks the command to be executed, whether it's of special "isql"-ish syntax or not, it executes commands it understand and send the rest to the server, unchanged.

There's no way to mimic ISQL's behavior right on the server, without ISQL. However some files can be loaded via the DB.DBA.VAD_LOAD_SQL_FILE function.