fallback_speed for routes requests

[jjbringg]

Feature

I know that fallback_speed exists for table requests (though possibly a little buggy) in order to use straight-line distances for between points where a route does not exist.

However, for flows that use those table results to build a route, it would be helpful to have the same functionality present in /route/ requests. The result's leg should have distance/duration based on the fallback speed, and the geometry (polyline or geojson) should just have a straight line connecting the points.
Attached is an extreme example geometry (manually created with a straight line in the middle of the route)

[jjbringg]

Is this something that's even possible, @danpat ? You did the fallback_speed for table reqs, right? I'm not super familiar with the optimizations done under the hood that might make this more difficult than the table version.
Thanks