Although the vaati were the ones who died and sacrificed to put Miska in check, the real counterpart to the Queen of Chaos on the lawful side is probably Asmodeus, who might be secretly aiding the PCs in their quest to reconstitute the rod and defeat the schemes of the obyriths. It would be interesting if you played that up, introducing some suspiciously helpful devils (who would probably be disguised erinyes or the like) and so on, letting chaotic PCs slowly discover a vast conspiracy on the lawful side of the Outer Planes, so the real solution involves not only reconstituting the rod, but also keeping it out of the claws of the baatezu. Perhaps the entire quest is part of Asmodeus' plan to rewrite all the cosmic laws in his image, and he needs the Rod in order to do that because of certain pacts and agreements he made with the vaati billennia ago.
Perhaps the baatezu could give the PCs a rite with which to unite the pieces of the rod, without telling them that it also teleports the rod to Nessus. So the challenge would be to figure out that their ritual is flawed and also to track down the true, uncorrupted ritual (perhaps in Mechanus). If they fail to do this, that's an opportunity for another quest to the Nine Hells to recover the rod from those who've gained it. Maybe Asmodeus' ruby rod and Dispater's iron rod are the eighth and ninth parts of the same item, and there's a struggle between conservative Dispater and ruthless Asmodeus over whether or not they should be united, giving the PCs an opportunity to play them against one another or even accept Dispater's help in thwarting Asmodeus' scheme.
While you're at it, why not bring in the eladrins and slaadi with their own agendas, to better showcase the diversity of Chaos? Maybe the eladrins want the PCs to bring the un-united parts of the rod to them so that they can twist its magic and change its nature before using it against Miska (maybe they want to redeem Miska instead of destroying him; maybe Miska was once an eladrin who was corrupted by the Queen of Chaos long ago, transformed into a tanar'ri against his will - maybe he was a former concept of Queen Morwel, thought long dead by all outside the Court of Stars). Maybe the slaadi are working variously for the obyriths, tanar'ri, and eladrins (often the same slaad working for all three), and appear in human form trying to convince the PCs to help one group or the other. Of course, some slaadi don't care what the Queen of Chaos does, and just want to keep the rod because it looks pretty and stings them when they touch it in such a hilarious way.
