Paul,
  Try searching around the forum on freeswan  It should work for you, but I think you are going to have some performance issues.   Based on what I know about physics and geosynchronous orbits.... the signal has to travel roughly 72,000km before it bounces off a transponder and arrives back on Earth.  
This causes a 1/4 second delay.
Add a second hop into space to your next land station and you have another 1/4 second lost.  The encryption of everything VPN software and the various codec delays are pretty minimal when compared with a 1/2 second delay added to the overall latency.
I guess what I'm saying is your are probably going to have a nasty bottleneck caused by too much latency.  You may not like the results  
