How does a common-emitter rail-to-rail opamp output stage work? Is there a discrete example?
The reason one doesn't find "rail-to-rail" complementary collectors drivers is that its hard to balance two current sources without accurate sensing using current mirrors.

Unlike emitter followers that buffer base impedance Zout = Rbe /hFE, they are current sources which by definition are high impedance which is not what you need for a voltage source rail to rail driver.  Using negative feedback is less stable and chances for shoot-thru are too difficult to prevent.  

Most R2R drivers are CMOS but some are high current with NPN low side
In complmentary drivers, it is ok to have one current sink, or source if shunted by a complementary low Rce against a low Pch RdsOn  resistance, as per below. 

e.g. LMV32x X=1,2,4 2.7~5.5V  160mA 
 

e.g. PA50 100Apk 40A  power Amp 400W