Those system variables reflect the LAST modal state set through G code. They are informational only meaning they don't actually set the mode. However, we do read them for presenting the modal states on the screen. So changing them basically only changes what is displayed.
The Fanuc manuals do not specify that those registers are read only. But they do not state that writing to them has any effect either. So we don't enforce the read only attribute and we don't allow the modal states to be changed by them either. Besides, there is already an interface for changing the modal state for #4014. G54, G55, G56, etc...
I don't believe setting the offset with the #4014 system variable would work too well with the G54.1 style extended offset either. So I'm inclined to think that those variables really are for informational purposes only.
Also, a word of warning: Be careful with setting system variables with screen elements (DROs or text boxes) while a program is running. It may not have the desired effects because of the look ahead!
Steve