They're already serving a very niche market.
Not gonna happen. It costs millions to buy licensing for characters.
Clearly, it would be a bad idea for a number of reasons. It would divide their market, it would require a high tooling investment, it wouldn't seal around furry tails, it would be the wrong size and in the wrong place much of the time, most furry tails aren't urine-proof or washable, etc.Wouldn't that kind of defeat the purpose of a diaper...If you used the diaper with some fake tail, it could ruin it.
Its an idea.