My bathroom is surrounded by a fuzzy leafed species, planted specifically for this purpose, I imagine. Many people I talk to around here can still vividly remember before toilet paper. I have yet to fully switch over, but I am considering it, and am wondering about the implications for my composting toilet.
I think the only difference of using leaves compared to toilet paper is that you'll be adding more nitrogen to the mix. This means that your C:N ratio will change, but you can solve this easily by adding more browns (carbon).
Some examples of good browns are straw, cardboard or sawdust. You can check the table in this answer to see the C:N ratio of some common household wastes and see what else you can use. Higher numbers mean more carbon.