Is 0.4 sec the maximum value for deltaTime?

It would be pretty easy to do if you’re happy rebuilding the engine from source. Just override UWorld::Tick I guess.
Whether you can hook in at that level without a source build, or specify your own UWorld class, I don’t know. I think you can specify your own UEngine derived class, so it might be doable.

And they really need to change Answer Hub settings to stop marked answers from being reset any time someone adds a comment!