You are correct, of course: if your application needs 2x RAM and you only have 1x, then there is no way mlock can magically save you from swapping. What it gives is protection from a noisy neighbour.

The good thing is that mlock/mlockall does not require elevated permissions. But it is bounded by the memlock rlimit, which you need to configure for the container. If you don't want paging/swapping, set it equal to the total memory you give to the container and call it a day.

I'm curious about the technical implications. Which ones do you have in mind?

To be honest, I don't quite see how mlock makes it complex. It's quite the opposite: it has very clear semantics and makes reasoning about system behavior simple. Disabling swap is the opposite: you're making a bet and hoping it works.