This is not an issue but rather something I would like to bring to your attention.
I've been working on an auto-ensemble mode using the playlib features you mentioned to be able to multibard without having to keep pressing ensemble mode after each song.

I've also been trying to compensate the plot for the ensemble delay (seems to always be constant regardless of latency) and I more or less succeeded in doing so by offsetting the notes in the graph while in ensemble mode and drawing a second guidebar behind the regular one that actually syncs with the sound of the notes.
As much as I like the concept, I am far from happy with my current implementation but I've been struggling to make both guidelines act the way I want.

This is highly WIP and I am completely inept at UI design but it does work realiably so far.
Maybe you can get inspired from this little experiment of mine so I welcome you to take a look: https://github.com/bilenkonito/MidiBard/tree/ensemble-experiments
Example of the feature at work (the video was captured a few days ago, I've already fixed some of the issues with the graph): https://www.youtube.com/watch?v=DlVFMZmkupo