How to activate Unity5 free version when behind a firewall / no outside internet is present?
I dscovered the answer to my own question:
step 0: Disconnect you machine from your internal network and start the Unity installer.
step 1: It will ask you some basic questions and then open the licensing dialog to load a license / save your license request.
step 2: Choose the save option and the Unity installer will create a Unity_v5.alf file.
step 3: Next use your browser to get to https://license.unity3d.com/ from a computer with outside access and submit the Unity_v5.alf file.
step 4: Unity services will then generate a Unity_v5.ulf file that you need to load into the Unity installer and the install should finish up.