In (Steam Directory)/config, open the loginusers.vdf text file and change SkipOfflineModeWarning from "0" to "1". You might also want to change WantsOfflineMode from "0" to "1". Then, build a Startup folder shortcut that launches Steam as follows: "Steam.exe -offline -silent"
After updating loginusers.vdf, you'll want to make it read-only or Steam will set the parameters back.
A more elegant approach that doesn't require setting the loginusers.vdf file to read-only would be to write a Steam start-up script that edits the individual parameters each time before launching Steam. That way, other user changes made within the Steam UI will stick and interactions with Steam outside the start-up script will operate completely normally.