Drill down on "outside the universe," as it seems to conflict with the definition of universe we're using here. Presumably, we mean something which has access to the nodes in the connected fabric of causes and effects, but is not an input to any of them- much like the proposed consciousness. If consciousness is caused by neurons, it's in the fabric, and our omniscient observer will be able to see whether or not consciousness is a node in the fabric or not. We'll avoid the infinite regress by enforcing that all observers are inside the universe / the connected causal fabric, but they are differentiated by where they are in the fabric.

From inside the fabric, we can only comment confidently on the nodes we have access to, and implicitly the nodes which are ancestors of those nodes. Agents like us are both causal ancestors and descendents of each other- we see clear feedback loops that convince everyone of the reality of other agents (barring those confused by solipsism, of course). There could be silent observers (of the consciousness variety or otherwise) that are descendents but not ancestors of the nodes we have access to- and because they're not ancestors of the nodes we have access to, we can't comment confidently on them! But by Occam's Razor, it's generally better to not put such nodes into your model of the universe, since they don't give you any additional predictive or manipulative power, but still cost cognitive resources to lug around.

(It seems to me that a principle that only relies only on the structure of the causal graph which excludes silent observers will also exclude the spaceship that has passed the horizon- and so either we need a principle from outside the model, like "my intuition says spaceships don't disappear but silent observers aren't real," or to put them in the same bin.)