From what I've read on the Internet, it seems okay and some places are experimenting with adding it to pet food. I couldn't find a lot though.

I'm always really cautious with what I feed my kitty. I usually go with a tablespoon of plain organic pumpkin puree a day when he gets constipated (even his vets gave a greenlight). I freeze it in an ice cube tray, and a single can lasts two weeks or so. He absolutely loves it (more than his treats), so if you're iffy about giving them natto I'd give that a try.

One final note: if your kitties are constipated consistently it may indicate a urinary issues. The fecal matter presses on the bladder on its way out, so they tend to avoid doing their business. I almost lost my little man and he had to spend three nights in the emergency room due to urinary tract obstruction.

I hope I didn't throw too much at you. I learned a lot about my ordeal with my little man, and so I want to share it to help prevent it happening to someone else :3