I have never seen such a feature before, but it should provide an interesting gameplay opportunity.

So yes, in a multiplayer/real-time environment (imagine FPS), how could I implement a slow motion/bullet time effect?

Something like an illusion for the player that's currently slo-mo'ed. So everybody sees him "real-time", but he sees everything slowed down.

Update

A sidenote: keep in mind that a FPS game has to be balanced in order for it to be fun. So yes, this bullet time feature has to be solid, giving a small advantage to the "player", while not taking away from other players.

Plus, there is a possibility that two players could activate their bullet time at the same time.

Furthermore:

I'm going to implement this in the future no matter what it takes. And, the idea is to build a whole new game engine for all this. If that gives new options, I'm more then interested in hearing the ideas.

Meanwhile, here with my team we're thinking about this too, when our theory will be crafted, I'm going to share it here.

Is this even possible? So, the question on "is this even possible" has been answered, now it's time to find the best solution.

I'm keeping the "answer" until something exceptionally good comes up, like a prototype theory with something close to working pseudo code.