Oh, really? I cant say I understand why that is the case, but i'll take your word for it.

In that case I should be able to use even higher resistor values (to reduce the wasted current) without violating the PIC's specification of a 10k maximum impedance on ADC inputs?

Something like:

Battery +ve ------- 47k Resistor ------- ADC Pin ------- 12k Resistor ------- Ground

= 9.56k input impedance ~ 10uS ADC acquisition time (and only wasting fractions of a mA of battery current)