I'd probably say "Unemployed people will increasingly vent their frustration, as their forebears did in the Industrial Revolution". But if you want to do something like your #1, then indeed, "was the case" would be better. I don't care so much for your #2, because there's a parallelism problem with the comparison implied by "as".

You are talking about something that occurred in the past, so you would want to match that. "Unemployed people will increasingly vent their frustration, as was the case with their predecessors in the Industrial Revolution." So, you can think of it as.. their predecessors are not currently frustrated! Here's another example: "Unemployed people will increasingly vent their frustration, as IS the case with the unemployed in other countries." This one is talking about something that is present and current. Although, I'm not sure if it's true or not ;-)

So there is no problem here with using a Present Tense with a reference to the past?

The first one is perfect! Just instead of saying 'forebears', predecessors is a more accurate word to use in my opinion