Can I buy license keys outside of Google Play?

Yes you can purchase licenses directly. It works like this.

1. You tell me how many license
2. You tell me to whom I should email the invoice
3. You tell me the Account (Google, Facebook, Email, etc.) that will be used to activate the licenses on the devices
4. I send the invoice using paypal.com
5. You pay the invoice
6. I setup the Registration server and you are good to go

To activate licenses you do the following:

1. Add the Account to the device
2. Download  Video Kiosk from the Play Store
 OR side load the .apk if you don’t have the Play Store
3. Open Video Kiosk
4. Touch/Click the button titled Activate

After that the Account is no longer needed, you can remove it from the device, leave it on the device, put a different id on the device, even completely remove the device from the Internet.