The attached VI will do what you want, using events. It's a bit more complicated than just catching the mouse click event, as you presumably want to click a second time to set the insertion point.
The behavior is obtained by setting a boolean called Entered in the Mouse Enter event, but only if the control does not already have key focus. Then, in the Mouse Up event, if Entered is true, we select all text; if not, we do nothing, which result in setting the insertion point.
This seemed like a good idea, so I filed a suggestion to have LabVIEW add a configuration selection to controls to do this (so no programming would be required).
Enjoy,
Roy