Looking at the game log, it seems that Sirius and Q are finally attacking Mavs, concentrating their attack on the Mavs Home Island. So I don't know if we need to encourage them to fight anymore.
But maybe a note of Congrats are still in order... what do you think?
We also need to Exchange Maps with Sirius to see if they are razing or keeping the Mavs cities. Hopefully razing, so that they have to spend time resettling. Probably razing, because I doubt that Q and Sirius could reach an agreement on sharing all those wonders. Those two teams seem very stubborn in negotiation for land.
Mavs are counterattacking the city Sirius just took with a large force, so this should be very interesting. Perhaps they have a long bloody struggle on their hands. Also, Mavs are building a big stack on Comet. I am hoping they plan to use it to invade Bode, rather than assault Nabaxica again, as Marthaton is now lost to them. If Mavs can bog Sirius and Q down a little this might give us the chance to catch up. Either way, Quatronia is going to have a tough time getting rid of The Mavs on Comet.
Finally, I would say offer the Exchange right away, or right after we capture Turnpike. In the in game message/diplo window, you can put some thing like...."As per our treaty the Klepto-MAZON" so they will remember that Marthaton is ours and the other land is theirs. Better to do it sooner, before they start making plans for the city.