NO. You can't do that (at least not without some fairly specialized equipment and procedures). 25-30kV is not to be trifled with, and without a proper load (the tube and yoke), everything will be so far out of whack that your measurements will be useless, anyway (not to mention probably destroy other parts of the board).
If you can't take the measurements with the monitor in the cabinet, the best option is to just remove the whole monitor and run it on a bench. I'm assuming you don't have an identical tube/yoke to test with on your bench, at least.