My random thoughts on this idea:
1) Imagine a scenario where you are flying past a mountain peak - make a 90 degree turn right there and it would be a short glide before ground intercept. But if you continue on course for a miles or so before turning, maybe you could make it to the other side. So would you highlight the other side of the mountain? Or would you highlight based on a simple perpendicular course from the route? Similar issue if you are climbing...or if your route crosses itself...the highlighted area would be ambiguous.
2) A customer at Oshkosh was telling he flew straight across Lake Michigan, and he manually calculated based on his altitude & glide radio that there would only be a 10 minute stretch where he couldn't make land. It occurred to me a feature like this might make that task easy. Then 10 seconds later it occurred to me that it wouldn't help that guy at all, since the glide distance ahead/behind wouldn't be visualized - just the side to side.
To accommodate both of these scenarios, perhaps it would be better to visualize the glide distance from any given point along the route. And perhaps a slider so you could move your point along your plan.