Hi gang! Doctor here, trained at and still work at the local "Rectal Foreign Body Center of Excellence", so I feel somewhat qualified to give my professional opinion.

Yes, nothing without a flared base should be used in this fashion. BUT, there's pretty much no risk for harm here. Mechanical obstruction is unlikely because, as OP says, it'll get mushy and get pooped out. Bananas aren't much of an irritant like a citrus fruit, so not much risk for chemical damage. Someone else said there might be a risk of potassium overdose, but not really. The rectum does absorb, but not as much as the stomach. So while some potassium will be absorbed, this is at worst equivalent to eating the same number of bananas. Which won't harm you.

So not a big risk here. That being said, flared bases, everyone!