Why would you want to cook your pork shoulder past the point at which it is done? Granted, that is a very forgiving cut of meat, and an extra hour at 250f will probably make little substantive difference. However, there is no shortage of methods you can employ to keep your pork at a safe temperature until serving. These include, but are not limited to:
- Wrapping in foil and storing in a cooler.
- Covering and holding in an oven at its lowest temperature.
- Using the "Warm" setting of a slow cooker.
You also could alter your cooking temperature so that it takes longer. However, I tend to think you're better off targeting a time a couple hours before you plan on serving it, to give you a little wiggle room if things take longer than expected, and then having a "hold warm" strategy at the ready.