That's not as easy as it looks.
Some devices will actually *stop* charging when hitting 100%, while some will either keep trickle charging or bypass the battery and keep being flagged as charging.
This depends on the device, and on the type of charger.
Unfortunately, the only devices where we can have (halfway) reliable info about these sort of details are Mk. >= 7.
So, no, not going to mess with the already fragile Kobo suspend code for something like that

.
(
This was already messy enough to deal with in a vacuum, so, no thanks).