I agree, the kV rating is hard to come-by on brushed motors.
I found some data to estimate the kV for GWS brushed motors at:

http://www.bphobbies.com/view.asp?id=A0320095

http://www.bphobbies.com (under motors) in general has a lot of data on
Feigao brushless motors and GWS brushed motors.

http://www.gwsus.com/english/product/product.htm
also has data on GWS brushed motors.

One thing about the voltage rating on a brushed motor:
it often seems to be not a single absolute number.
It seems to be more of:
"if you run at this voltage with this propeller, it will draw this current (which if too much, will overheat the motor)"
So each propeller/motor combination will probably have a certain maximum allowed voltage
(which may depend in part on how much air flow the motor gets)

So it is not obvious to me what the relationship is between the voltage rating and the brushed motor's speed.

One detail on the voltages in these charts:
these seem to refer to the actual voltages that the motor sees,
as opposed to the rated voltage on the battery, which as you may already know, can be quite different
(for example, I have seen my 9.6V NiMH batteries produce ~8.5V under load).