Windows 7 x64, 4.9 binary release
Below is a screenshot of a vector curve with X and Y both hidden and locked. Where curves overlap, the only action available is to add a key to X, even after clicking Z. Even when all curves are hidden and locked, you can still hover over the blank space where curves would be and add keys. In this example, all curves have a key at 0,1 and 1,1.
Hey OhiraKyou,
Thank you for taking the time to create a post for this issue. I was able to confirm what you are reporting and have gone ahead and entered a bug ticket for this issue (UE-18554).
Once this issue is fixed and implemented, it will be added to the release notes within the Hotfix or Official Release for its respective engine version. You can find the full Release Notes in our Documentation found below.
Let me know if you have any other questions.
Cheers,