Yeah, D1 there is no realistic (legal) under seat storage space. I've gotten away with having a backpack or wrapped pillows/blankets under the ottoman at times in the past, but I wouldn't rely on that working with a pet carrier.

I'm not really sure why they would say you can't do it in PS. The storage space under those seats should be pretty similar to domestic F. The only thing I can think of is maybe the footrest gets in the way?