Also, it worked fine for me using Python's built in development server. If you have python extract the offline files into any empty folder, open it in a console, and run:
python -m http.server
Just make sure to kill it when done and definitely do not publicly expose it.
I tried this (put HEN_UPD.pkg at the root of the same USB I used to install the HFW) but it still tries to download from the internet. When it tries to install the downloaded file it fails and reboots. Does it need to go into a folder on the USB? I'm self hosting the offline installer.
EDIT...