Offline license signing is a solved problem, Sophos for one has figured this out with the way they license their UTM product.
When they give you a license file, it's cryptographically signed with their GPG key, and the public key resides on the appliance for verification. All you have to do is get that license into the system, either by USB key, typing it in yourself in Vim, or simply uploading the license file in the webUI if you have access to it.
When they give you a license file, it's cryptographically signed with their GPG key, and the public key resides on the appliance for verification. All you have to do is get that license into the system, either by USB key, typing it in yourself in Vim, or simply uploading the license file in the webUI if you have access to it.