If you can’t cost-effectively or cosmetically get a direct network connection to the digital signage player, what viable alternatives exist?”
Wired networking is always best, but if it’s not possible, utilize Wi-Fi. Wireless AC and AD are much longer range and higher speed at a much greater reliability than ever before. I used to be against using wireless for critical applications, but as long as wireless reception is good, there is no reason not to. Mesh systems are quite established enough to blanket any space in wireless within reason… and proper security will keep out all but the most dogged intruders. Personally, I’m not yet a big fan of cellular for speed and reliability, let alone cost… but in extreme need, 4G LTE can serve in a pinch for applications like vehicle-based signage.