I'll start with the JMX part of the question first.
Looks like the maxQueued attribute is not exposed.
As for the rest of the question...
We had too many issues with ArrayBlockingQueue, we removed support for maxQueued and the ArrayBlockingQueue from QueuedThreadPool starting in Jetty 9.
It started out as a performance question, then part of our mechanical sympathy efforts.
Eventually, some performance tests were run ...
And we determined that the ArrayBlockingQueue performs some harsh JDK locking that actually hurts performance.
So we removed it as part of our Queue/ThreadPool cleanup
However, we also made Jetty 9 use the built-in java.util.concurrent techniques.
Which allowed us to even expose that entire framework as an option for developers to use as the ThreadPool for jetty.
The wiki about high load doesn't mention the maxQueued configuration for QueuedThreadPool, as that's not related to high load in our minds.
It would be like trying to improve the performance of an F1 racer by changing the length of the spark plug wires.