I would start with a covered cat box. I'm not sure how to address this problem altogether but a covered cat box will keep most of the spray inside.

Have you tried different cat litters. It could be that the cat doesn't like the way the cat litter feels on her feet or the smell of it. There are many different kinds. I use Tidy Cat but I am preparing to switch to World's Greatest Cat Litter. One of my buddies uses some kind of natural material. I have dropped her an email to ask what and I'll come back and post it when I find out.

Another option might be two cat boxes. Many cats do NOT like to share a cat box. In some households you have to have one box per cat.