When was the Laud O Zion sequence added to the mass?
When the feast was instituted in the 1200s. The sequence and hymns for the feast were all written by St Thomas Aquinas specifically for the feast.
The Pope commissioned St. Thomas Aquinas and (I think) St. Bonaventure to write the prayers of the Mass of Corpus Christi after the Feast was first approved (it took a while! But Our Lord had appeared and asked for it, after all :D). The two saints wrote their separate ones, and on the way to present them to the Holy Father, they traded and read to see what the other had come up with. After trading back, St. Bonaventure tore his parchment to shreds, in all humility, and said, yours is by far better!
It’s a good thing St Thomas didn’t do the same!