For proposition P: If $b^2=ac$,
we can assume $a=b=c=0$,
which obviously satisfies the condition, but they do not form a geometric sequence,
thus, this proposition is false.
For proposition Q: "The function $f(x) = \cos\left(\frac{\pi}{2} + x\right) = -\sin x$ is an odd function",
thus, proposition Q is true.
Therefore, $p \lor q$ is a true proposition,
hence, the correct choice is: $\boxed{A}$.
By determining the truth of propositions P and Q, we can judge the truth of the compound proposition.
This question examines the judgment of compound propositions, trigonometric functions, and sequence problems, making it a basic question.