Does module_invoke() work for non core modules?
You are probably looking for module_invoke_all. The module_invoke function only invokes a module in a specific module (casrel.module in your example).

By the way, you usually do not need to invoke hook_nodeapi yourself. The node module invokes hook_nodeapi when necessary, if you just use the api functions (like node_view) correctly. If you need to alter a node before it's viewed, you need to implement hook_nodeapi instead of invoking it.