Looking at the prices of castellated holes it seems a lot quicker and easier to just use it basically as a QFN style package, with all the leads underneath and solder the top one. Would it work to put down paste, heat a small adapter PCB with hot air to solder it onto the main PCB and then hand solder the op amp on the adapter? I assume the enough heat would get to the paste to liquefy it without damaging the adapter board but maybe castellated is safer.