My only complaint is the mix between snake_case and PascalCase. I tried removing it(in favor of snake_case, but you could probably get a similar result if going full PascalCase), and it looks much better(no guarantee it has the same semantics, I've never worked with rust):

     fn handle_message(&mut self, find_node_query: messages::find_node_query) {
        let origin = node::udp_node::deserialize(find_node_query.get_origin());
        let target = address::from_str(find_node_query.get_target());

        {
            // Borrow self.routing_table first time
            let nodes: vec<&box<node::node>> = self.routing_table.nearest_to(&target);
            let response = message::response(
                self.transaction_ids.generate(),
                response::find_node(&self.self_node, nodes)
            );
            origin.send(response.serialize());
        } // Done borrowing self.routing_table here

        // Borrow self.routing_table again
        self.routing_table.insert(origin);
    }