Jim,
You can basically do that with the 250nm ring now by zooming out, then add a waypoint at the intersection of the route line and 250nm ring. The iFly will pop a list of optional airports, fixes, etc when you drag to that point or touch to add waypoint. You can then zoom in close and rubberband to a differerent airport in that area if you like.
Sorry if you misunderstood, it's on the WishList. There are Distance Rings in V7 (1nm, 2nm, 5nm, 10nm, 50nm, 100nm and 250nm visible at various zoom levels) but not a custom distance option. Your Fuel Ring description is what I picture as well.