Climbing is possible in the National Park. The routed is not prepared though can also be climbed as top-rope using two ropes.
From the town of Somoto it's a 15min bus ride to the second entrance of the national park. From there it's a 1km walk to the gate of the NP, and another km to get to the canon. The route is to your right, you don't have to cross the river. (Swimming in the river is fun though)
Guides offer (eco-tourism) climbing and provide equipment.
In Somoto, ask for Gonzalo Hernández Cáceres.