i've never done it but it would not be too difficult for someone who wanted to do it.
use this schematic for reference:
http://www.cbtricks.com/radios/uniden/washington_mb8719/graphics/uniden_washington_mb8719_sch.pdf
D24 is the ANL diode in this chassis, and basically the way the circuit works is that with the ANL on, D24 is in series with the circuit. when the ANL is switched off, D24 is shorted out and replaced with a capacitor, C93.
so you could trace the wires back from the ANL/NB switch, and find the one that goes to the anode of D24, and the one that goes to one side of C93.
pull those wires off of the back of the switch, and put them on their own switch.
or you could permanently short them together to keep the ANL OFF.
you could also just insulate the ends of the wires, leave them separate, and that would leave the ANL ON all the time.
LC