I'm not sure, but I think your only option may be to do some calculations. The mouse down on the array will give the coordinates of the click in VI coords. You can get the coords and size of the cluster and the coords and size of the string using their property nodes. Then, you have to calculate whether the click was on one of the visible strings using addition, subtraction and quotients. Once you make a subVI out of this, it should be easy to use.
You should watch out for the index display and the label, as they change the values of the coords. I'm sorry, I can't think of any easier way to do this.
Correction - I just thought of one. If you use the KeyFocus property on the string inside the mouse down event, it seems to work. I'm not sure whether this will work when bringing the keyboard to the front, which is why I left the other suggestion in place. Here's my test (7.0)
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