I think Luxu is meant to become the eye of the future that let the MoM see into the future. Take a moment we know at times people who are the same being can share memories, can share emotions, who is to say they can't share sight as well? I think DDD was hinting that they can "Ansem possessed Riku enabling him to see through Riku's eyes in real time" was a bit of a line that didn't make too much sense in regards to how Xehanort knew the future. What if he meant that literally? What if when you possess someone your eyes literally become linked, you become able to see what they see....it give a whole new meaning to people getting Xehanort's eyes when possessed. It may be posession literally would turn people into cameras for the main computer/body which be Xehanort, then it just be a matter of learning to access the video feeds if you will.
Now let's apply this to MoM and Luxu. MoM sheds his body and joins with Luxu, Luxu is now his vessel through which he can watch the world. Luxu travels to the world of the future and ends up linking with Xehanort. Then Xehanort possess lots of people and what they see feeds back into Xehanort. Xehanort is the main computer, his vessels are his cameras, and Luxu/MoM who is linked with Xehanort acts as sort of a storage. A storage the past version of the MoM becomes able to access, enabling him to see the future in real time, and allowing him to write the tome of prophecies as well as set everything into motion.
And before someone says this is too complex may I remind you of Sora's ability to wield 2 keyblades? He got that ability via his magic clothes which let him access Roxas's abilities within him. Roxas awoke that ability through Xion, specifically it required Xion and memory of her keyblade being erased from his memories. This prompted him to force those memories awake again which in turn didn't just wake his memory of a second keyblade, it woke the literal second keyblade within him. Which was only possible because he had Ven's heart, Ven being another keyblade wielder his keyblade was stored within. AND the only reason he had Ven's heart is because back when Sora was first born he connected to Ven's heart repairing it and later that link enabled Ventus's heart, which was damaged a second time, to migrate from his body to Sora's.
Isn't funny? How we thought the Master of Masters was good? Now we know for sure he is the main villain and caused the Keyblade War?