I did a little bit of playing around and I don't think what you want to do is going to be possible with the way this component is working.
I have a script that does exactly what you want increase by the increment, in this case 5, and when I hit the up arrow the script does write to the value property, but the actual entry field still displays a value of 2. I imagine there is an internal mechansim on the numericEntryField spinner that has the arrow keys increase it by 1.
When I click off of the entry box or hit enter and it is supposed to accept the value it reverts the value in the props tree to be the value actually in the field itself.
