What kind of species is this for? Coir holds better humidity if that is needed. You can always give him a soak or spritz with some water. They will usually use a leg to rub it out. But to answer your question, reptile bark is fine but not for humidity.
I mixed my coir with organic soil and sphagnum moss at about a 2:1:1 ratio...unless my Hermann's, Littlefoot, has REALLY been digging, he doesn't get very messy, though it sticks in his toes and leg scales a bit sometimes. The soil helps weigh the coir down. He's pretty good about using his legs to make himself presentable too. If I notice he has stuff in his eyes, but isn't cleaning himself up, I usually just stick him in his water dish for a second, and he rinses off. Just some options other than switching substrate.
I'm not sure how old your guy is, but humidity is important for young torts.