Hopefully whatever it was shows up on some DVD of the rehearsals, I thought I read on here they filmed them in case the tour never happened because of the pandemic.
They filmed the final "show" in it's completed form on the last two days of rehearsals when they were ready for the stage.
The earlier rehearsals, according to Nic, included up to 40 songs of which about 25 have made the set. I would imagine that several of the 15 songs that didn't make it were dropped quickly and not thoroughly rehearsed. While there may be footage of the rehearsals, we'd likely only see fragments in an extra on a disc.