That would only work if uploaders actually place the chapters in the correct volumes. A lot of them just upload as "No Volume"; either because the volume the chapter would be in hasn't been published yet and they don't go back to fix it when it is, or that they're too lazy to go to Bookwalker JP or Amazon JP, and check the Table of Contents via the free preview to confirm which chapter belongs where.
Yes I've seen that a bunch, and that would be OK with me.
So it could change the selected cover art for me based on my last read chapter (that has a volume assigned to it).
Often a scenario is something like this:
Volumes 1-6 do exist, and the cover art for all have been uploaded here. But translations have only completed for volume 3, with like a few more chapters unorganized, without the volume defined.
Say I've read every chapter available: In this scenario, the current site code would show me Volume 6's cover art, but in my suggestion it'd show Volume 3's cover art, even though I am technically reading what will eventually be defined as Volume 4's chapters. I am OK with this.
