This could be added to the civ tech tree.  This would possibly make the diplomacy system more robust.  This would allow you to get your allies and/or enemies to do things they might not normally do due to race relations.  This is esp. useful in locked teams. 

I recall playing a game with locked teams (3 v 3) and at one point my original ally asked me to attack my other original ally.  However, I had no such ability to do that.

Back channels or shadow govt's could involve embassies (again) or the envoy cruisers.  Perhaps this back channel or shadow gov't involves resources spent since you would have to 1) pay off a lot of people and 2) covertyly have the infastructure in place to support this.  Also, espionage (as strongly requested) could be a part of this "shadow gov't." 

After all, shadow gov'ts don't play by the rules.

Here's just one idea for the shadow govt side: imagine 3a v 3b locked teams.  Naturally approval among team 3a is 100.  One of them is getting a little too big for its bridges (sp?). 

tech 1

shadow gov't  >> opens up recruiting ability via envoy cruisers.  [politicians, movie producers, generals, actors etc]

tech 2

methods >> a) generals slow to implement orders

                   politicians introduce legislation ambivelent to your target\

                    c) entertainment makes movies towing your agenda against your target

                    d) actors preach your word in tabloids against your intended target

tech 3

subterfuge

                   a) target planet hit (destruciton of structure) by you and blames your ally

                    your have the ability to have sabetoures originate from your ally to attack the other

                   c) gain the ability to see who is trying to target whom!

                   d) steal artifact (added later)

you get the idea.  the great thing is you can do this to your enemies too!!!  They may have zero approval towards you and zero pacts, but you can open up something that gets them fighting eachother.  the old divide and conquere.  Also, when someone opens and embassy (quite visible yes . . perhaps it is a building like the red cross) on another planet, the owner is not known yet per se.  They could be named like the millitias at the beginning of the game that occupy the grav wells of available planets.