As far as I can tell, a placed XControl does not respond to attempted font changes, including Ctrl-Plus, Ctrl-Minus, or selections from the Font Selection menu. The label will change, but not any text inside the XControl itself. It also appears that no events are generated on the XControl for these actions.
I would like an XControl to know about these changes in fonts, either automatically applying them (with corresponding height resizing), or generating events with this information.
National Instruments will not be implementing this idea. There is no further feature development planned for XControls.