@gernot, you're on the right track!

First, in the new auto_loader, you'll instantiate your class-override as the 'opc' session value. Since that will occur at CP-74, the instantiation of the base OPC at CP-75 won't occur, since the auto-loader won't overwrite that class if it's already there.

For the class-inheritance, you're correct ... you'll only provide the methods in your class that are being overridden, with the same names and inputs as the 'base' OPC methods. You might take a peek at my Database I/O Manager as its /admin/includes/classes/dbio/DbIo*Handler.php classes use class-inheritance to provide handler-specific actions while using common/base functions within the DbIoHandler.php class.