I'm assuming this is because of, um, splatter? If that's not the reason, disregard the rest of this post.
My only suggestions would be to spread your cheeks when you seat yourself and consider washing before wiping. You can take a dedicated water bottle in with you and using your arm as a sluice pour water down it and into your crack to wash things a little before you wipe. That's kind of a backpacking thing- check this site out, there's an illustration of what I mean down towards the bottom (it's a cartoon, totally safe).