our kittens must be related.
i had a litterbox next to the tub for months, and decided it was time to upgrade him to the 'bigboy' litterbox in the upstairs bathroom. that is when he first started using the tub. i took him to the vet, they couldnt find anything wrong. i wound up closing the bathroom door. he used the upstairs litterbox for a few months without any problems, but then found the sink and would use that to poop. what i have to do now is fill the sink with water. it fascinates him and he still tries to get up there, but is deterred.
i have also found out that he forms crystals in his urine, which can become a serious health problem. i know cats try to "warn" their owners by peeing/pooping in different areas of the house. is there a connection? i cant be sure, but that's my story.
i hope this helps a little bit. at least you know you're not the only one with a tub pooping kitten.