I don't have a reference, but as evidence of the constant pressure side, I can say with some conviction, based on readings from my fuel pressure gauge I have between the lift and Injection pumps on my '84 300D, that the pressure stays at all engine speeds between 10 and 15 psi. This range appears unaltered whether I have the pedal fully depressed or am at idle. It seems logical to me that if it were a constant volume supply that the pressure would be greatest at idle when the demand from the IP was lowest.

Probably not definitive, but it's logical within my limited scope of knowledge.

Ben