Hello,
we also had this problem, our first idea:
Go to the glazier, have the missing piece cut, and paint the back white.
But then it didn’t look exactly the same as the original IKEA wall panels.
In addition, we backlit the panels, so you could see that the "hand"-painted piece of glass looked very uneven.
I just didn’t manage to get the white paint layer perfect.
That’s why I thought: if you can already see a difference between the "patched" glass pane and the original IKEA wall panels, why not make a virtue out of necessity and emphasize the difference:
So plan B: paint the glass pane with blackboard paint. Now the piece deliberately looks completely different and you can write recipes, little notes, etc. on it...
Those were our ideas... does anyone else have others?