Is it possible to make this recipe with a bone-in leg of lamb? How would I need to change the cooking time?

You can but the method calls for rolling the boned meat around the seasoning mixture, essentially stuffing it, so you'll either have to make some adjustments or skip the step entirely (which would create a significantly different outcome). To take advantage of the bone (a good thing in my book), you could open the roast up as if to bone it but leave the bone attached. Bone-in roasts are harder to carve but worth the effort if you ask me.

Cooking time will be extended somewhat so you might want to give the potatoes a little more head start than called for (the final temperature will remain the same of course).