Relay-assisted satellite networks rely on multi-hop architectures that expose data integrity to malicious or compromised relays. End-to-end Message Authentication Codes (MACs) avoid trusting relays but delay error detection until the destination, incurring costly retransmissions over long-delay space links, while hop-by-hop integrity requires trusting intermediate nodes. We propose a superposition-coded integrity scheme in which the source embeds the end-to-end MAC tag directly into the physical-layer waveform, allowing the destination to recover the tag independently of relay behavior. This prevents tag forgery even when a relay possesses the end-to-end key and eliminates the need to forward authentication data through relays. We validate the approach using a three-node software-defined radio testbed and show improved authentication reliability under malicious relay scenarios.