How about implementing k-shortest path algorithm?
I searched about the problem a bit and found this website: http://code.google.com/p/k-shortest-paths/
It uses Boost and C++ and it's GPLv2.
This is also something to consider for alternative paths:
Here is another K-Shortest path implementation that does not use Boost:
Is seems that patch #75 is implementing this.
I have merged a KSP from GSoC student into branch sew-devel-2_0
It was not from GSoC but from Dave Potts, I think ;-)
Right sorry about that.