Well, you could do it. Just connect the base of and NPN to positive supply and both the emitter and collector to ground. In this case you would be shorting the supply with two parallel diodes, and your transistor will not last long.
In a normal configuration, the voltage on the collector is going to be a little higher than that of the emitter even at saturation (Vcesat) and they will not both be forward biased by 0.7V. I guess the is why Vcesat is never zero, get it?
Bob