Variable Quant + Variable Bitrate (Can it happen?)

Would someone please take a bit of time to explain to me why there 
can't be a variable quant with variable bitrate mode?
I know, I know - it exists (2 pass right). But that's not what I mean.
To my understanding 2 pass takes the input bitrate as a avg it intends 
to meet. It then analyzes the video determining what parts of the video 
need what bitrate then on the second pass varies the quantizers as need 
to achieve that overall bitrate (more or less correct?).
It's been stated that 2 pass has a method of determining what level of 
quantization a macro block should have (to retain high quality).
Could this not be determined "on the fly" during a single pass encode 
to allow instead of a fixed quant but rather a variable quant + 
variable bitrate mode? Which would use the lowest needed quantization 
for a macro block while still allowing for higher ones to be used when 
possible.

This way it is possible to have a high quality single pass fixed quant 
2 without wasting bits.


Thanks for your time in advance,
Lee