yeah it really depends on how they designed the charge controller. it could be fine, it could die a slow heat death, it could just asplode. balance would not be my main worry here ;-)
(also aren't these "smart" batteries with their own electronics/management too?)
on something like an apc ups where the specs are known and/or the thing is smart enough to know better (software), I've not had problems adding on extra or bigger batteries. same goes for my solar backup and electric bike projects recently. but on the device we all love and hate...?! hmm... yeah...