Thrawn wasn't there, but some stealth lore from the EU makes it clear that he had a part in planning the Battle. There's an illustration by Joe Corroney of Thrawn standing with Grand Admiral Takel, Vader, and Palps with a hologram of the second Death Star, and it's clearly not long before the Battle, as Thrawn is openly wearing his Grand Admiral uniform. The Zahn novel Choices of One has him touring the DS2 construction site at Endor with the Emperor and providing tactical advice about the inevitable Battle, even Years in advance. Heir to the Empire from 1991, where it all began, has him losing his normal calm demeanor at Pellaeon when the subject of Endor comes up, and Thrawn really seems to be taking what happened fairly personally, as if it was his own plan that hadn't been properly executed. The Essential Chronology had at least two un-named Grand Admirals dying on the second Death Star, which was later changed in the famous Gamer article on the Grand Admirals to just Declann. Thrawn evidently didn't know about Declann being a master of Battle Meditation, given his lines in Heir to the Empire. Thrawn might well have planned to be present, but lingering issues with the Zaarin and Harkov betrayals probably were what prevented him from being present, along with the need to maintain the "mapping expedition" cover story of him being "exiled". Thing is, Thrawn didn't need to be present, as he only contributed some of the planning. Grand Admiral Declann was coordinating the Fleet via Battle Meditation, and three other Grand Admirals, though not necessarily their personal ships, were also present and not for any known reasons. It inevitably raises the question of why didn't Grand Admirals Takel and Makati try to rally the Fleet against the Rebels, and why did those two escape disgrace and punishment for fleeing the Battle. Had Thrawn been present, he wouldn't have fled, and surely would have tried to rally the Fleet, unless ordered not to by a higher authority. With Vader and Palpatine dead, only the Ruling Councilors (Sim Aloo, Janus Greejatus, etc) were powerful enough to give orders to a Grand Admiral. If they had been among his enemies in the Imperial Court, the might go so far as to try to kill him. It would be in line with their known prejudices and how they are described in the ROTJ novel. Thrawn, being an alien and one fairly unknown to most of the Imperial Navy, and also officially in a state of disgrace, might not have found many ship Captains willing to take orders from him if he could even get off the DS2 safely. Certainly, he would have a harder time than Grand Admiral Teshik, who himself was almost left to die on the DS2 by prejudiced Imperials who refused to help a 'borg. A non-Human, even a Grand Admiral, might not have gotten any such help. Assuming though that Thrawn does actually get off the DS2, he might not end up acting much differently than Teshik. Teshiks ship was probably damaged by the explosion of the DS2, probably the engines as it was trying to move away and barely did so in time due to the delay with almost nobody stopping to help Teshik. Teshik surely made use of the marooned TIEs from the destroyed or fled ships and whatever craft got off the DS2, plus made skillful use of his tractor beams and the wreckage of the Battle to create a debris shield around his ship to make it harder for the Rebels to get at him. He could also have made clever use of the radiation and munitions localized environmental effects from the DS2 explosion. Given that Teshik was later executed for war crimes, and that a lot of the Rebel fighters had to shoot down or destroy debris before it fell on Endor (per the Rogue Squadron comics), it stands to reason that Teshik was able to drag out the Battle in part by using his tractor beams to propel larger debris bits down onto Endor, in addition to propelling it outward at any larger Rebel ships that got close to his ship. Teshik might even have been trying to buy time for his repair crews to get his ships engines repaired so he could escape, and bring as many otherwise stranded Imperials with him to fight another day. Thrawn would have done no differently. But the Rebels had better fighters, and were able to eventually wipe out all the extra TIEs, and with the bigger Mon Cal ships clear away the debris shield around Teshiks ship, hence the line about it succumbing to overwhelming ion cannon fire, which would have killed his tractor beams and normal weaponry. Just as Teshik was remembered as a martyr and a hero who held off the Rebels, and as a monster by the Rebels, so would Thrawn have been remembered by history had he been present at Endor.
I am strongly of the opinion that if Declann's Battle Meditation remained uninterrupted, then the TIE/in of (iirc, Emperor's Sword Squadron?) would have dispatched Lando, Wedge, and anyone else making a run for the DS-II's power generator. After all, they were right behind them during a good portion of the pursuit.
