now I'm totally confused. The second gauge is definitely better in terms of the voltage range it measures. Can't I simply wire from the dimmer to the gauge to the light fixture? The current passes through the gauge, right?

It is just a marking on the gauge. Nothing to do with how much it measures. Just how that measurement is represented. If this is just for fun, just measuring some random voltage then there is no need to have a different representation. It is up to you.

 

Since I want it to behave realistically, I'm almost thinking I should buy an appropriate analog volt gauge for 110vac power and put the guts into a vintage housing. Then I could put it inline between the dimmer and the light fixture, right?

You can use the D'Arsonval movements you have with a photoresistor. You just need to determine what the full scale deflection current is and make a circuit that gives the needle movement you want.

The circuit below is a simple current source that uses an LDR (light dependent resistor) to vary current:

The meter goes on the collector of the transistor. The voltage divider is used to set the maximum and minimum current in Q1. Adjust resistor values to give the desired bright and dark currents with the LDR used. The values used give an emitter voltage of about 3-5V.

You could wire the meter to the dimmer switch you plan to use, but using a low voltage power source is safer.

 

here's a working gauge with a steampunk look already. Wish it didn't have a 300v range because the needle won't travel far but it's good to go:

You just need to change the shunt resistor to give the needle deflection you want.

 

FYI- I bought a modern analog voltmeter 1-150vac and an 0-30 amp ammeter so I won't need to mess with conversions and mismatched ranges. Thanks for all your help!