Whatever that rule is, I bet the mods at /r/interestingasfuck are ready to enforce it.

Good thing that the blackout is nothing to worry about, lol.

I mean, if all you are posting is John Oliver, it achieves three goals:
1- Puts the spotlight on the protest, which many users probably didn't know much about or didn't understand (cause they were out of the loop and just found reddit blacked out all of a sudden).
2- People eventually will get tired of John Oliver and/ or the same images will start getting reposted over and over again, which makes the sub uninteresting and users less likely to lurk or engage.
3-New users of the platform will come into reddit and see it filled with a bunch of crap instead of thought out content.

Since reddit is not playing fair there is no easy answer on what's the best way to protest. If they remain closed and they just put new mods in charge that will keep the sub running bussiness as usual, making the sub as unatractive as posible sounds like a better option.
I personally jumped ship and came to kbin, but I don't run a subreddit.

I assume the desired goal of the John Oliver thing is getting John Oliver's attention.

The outtakes for that episode are fantastic: https://www.youtube.com/watch?v=Bh7Nz4bIwss

The outtakes from that episode were just amazing. Ted calls Linda a "dick-infested man mattress" and I fucking died.