Opening everything and making it free during strikes sounds like a great way to do it, but I don't think the power that be would allow it.

I've never seen a barrier hopper stopped, but I'm sure the bobbies would be sent in if thry announced everyone would be doing it.

There's also the problem of disconnected services - the purple line is Rail, so while they use the same stations, they can be on strike when the Underground isn't. So you could go through open barriers at a rail station but you'd find the barriers shut trying to leave from an Underground station.