How can I keep a slow roasted pork belly roll warm while roasting potatoes?
It's normal to rest meat before carving, sometimes for quite a long time. 

The oven will already be hot after cooking the meat, even if not quite as hot as you'd use for potatoes. so I suggest you parboil the potatoes (often recommended anyway) and put the roasting tin and fat into the oven to preheat when the pork is nearly done. Then take the pork out of the oven, put the potatoes in and turn it up. The pork should rest under cover on the container in which it was cooked. A loose (tented) foil lid with a tea towel or two over the top is good insulation. If you want to speed things up further, you could turn up the oven a few minutes before taking the pork out. The effect on the pork will be minor (in most ovens) but the potatoes will cook quicker and may be more crispy.  I doubt you'll get very crunchy crackling at that temperature anyway.