We use pine bark for mulch where I live and it's never deterred the cats; however, the larger pine nuggets also used for mulch might not be as attractive. (I hate the nuggets because they float away in any kind of a decent rain.)
tmv - now those are some darn determined cats to poo on the doorstep!! :-D
A lawn and garden expert once said to try pine needles or anything else that is not as soft and welcoming to the cats. Pine bark, too, I think. But, some cats will go anywhere. We had a neighborhood full of some that wondered around in a previous house where we lived and the darn things would literally poop on the doorstep! Yuck.
Mulch certainly does make for an attractive litter box! Wish I could help on this one, but I've tried various commercial and homemade products over the years and none ever seemed to make much difference. I think the most effective remedy was putting a lot of ground cayenne pepper in the area. Of course, one rain or one session with the sprinkler gets rid of it and you have to reapply - at least it's cheap. Too bad we aren't neighbors; my dog would be more than happy to clean out your "litter box" for free! :-\