I would need to see the full circuit you are thinking of, but I don't think this will work as you think. Diodes are non-linear, not anywhere near "constantly proportional."
The voltage drop across a diode is a constant, typically 0.7 for a silicon diode. But, that is only true once it is fully conducting, which will require more than 0.7V. Below that point, it's response is a curve, not a line.
You propose this in parallel with the shunt. The shunt is of a very low resistance, and will hold the voltage across the diode very low, much lower than 0.7 V. The diode will not be conducting. Also, having anything in parallel with a shunt will change the resistance across the shunt and cause any meter connected to it to read incorrectly.
If you are going to use a diode, however you use it, you will need an amplifier to get the voltage from the shunt well over 0.7V. I would think something like coming from one side of the shunt, to an amplifier, through the diode, then an attenuator. Basically a pair of resistors, one in series, the other to ground. They would be selected so the attenuation exactly equals the gain of the amplifier. One of the resistors could be a potentiometer for adjustment. Then to the ammeter. The other side of the meter would go directly to the other side of the shunt. This would cause the meter to work in one direction, but not go negative in the other direction.
I have become unclear though on exactly what you are trying to achieve however. I am very clear you want an analog gauge, and for some reason the blue seas gauge that moves both ways won't work for you. Can you please restate what you are trying to achieve? What exactly do you want this meter to show you? Are you looking for a pair of meters, one that shows current in one direction, and the other that shows current in the other direction? The easiest solution might in fact be a digital solution, that drives 2 analog meters. Perhaps an Arduino or something. The shunt could feed an analog input to the Arduino, and the Arduino could decide what to do with it and drive the two meters. Or a single meter that always moves positive, and illuminates a charging or discharging LED, or any of a bunch of things that might be fun.