If you can’t cost-effectively or cosmetically get a direct network connection to the digital signage player, what viable alternatives exist?”
The answer here depends on many factors. If you have few content updates (say once per month) and few locations (say 20 or less) and the content is just a simple video, then by all means, send around a USB stick and have the store staff plug a new one in each month. Of course, you won’t really know if they’ve done that, or if the screen is on…but if your back is against the wall, it should work.
If you need weekly updates and have more locations or need real-time data integration, and getting a wired Internet connection is not possible, then I’d try either Wi-Fi or a cellular connection. Wi-Fi will be cheaper, but will require that you make friends with the IT department. A cellular modem such as the OptConnect Neo (http://www.optconnect.com/OC-neo-Wireless-Modem_8) available from Microspace (http://microspace.com/digital-signage-kiosk/) provides a small robust modem bundled with an inexpensive data plan that plugs into the Ethernet jack of your media player (PC or Android). There are no drivers to worry about (unlike USB cell modems). And it works really well. The only gotcha is that you need to have cell reception AT the location (just pull out your cell phone during your site survey and see “how many bars” you have!) And best of all, you avoid talking with the IT department all together! There is a monthly service fee per modem, but it’s gotten a lot more affordable in recent years. You do need to explain to your customer, however, that if there is no cell service at the location, and Wired or Wi-FI is not possible, then it’s back to “sneaker net” of USB keys or picking a new location for the screen.
I have also used power line carrier Ethernet adaptors in the past. If your Wi-Fi is flakey, or bumps you off (and forces you to login), this could be an alternative. Or perhaps just set up a secondary Wi-Fi network that is more reliable and doesn’t require a separate login.