I would have thought it would be possible to estimate the time of SR/SS from the Air Almanac.
Its easy to do on a ship to an accuracy of a few seconds from the nautical almanac. Obviously you need your estimated position at Sunset, but then how do you know when Sunset will be? So you need an estimation of when SS will take place. Which on a ship is easy as you can just check when it happened last night.
Obviously you can't really do this on an aircraft because of the speed. For an accurate answer I guess some sort of differentiation would be required. If you can make a reasonably accurate guess of where it will happen, you can work something more accurate out.
Or you could have a look at this:
SunTimes Freeware download and review - geographical time calculator from SnapFiles