The VERY old solution to this problem uses two resistors and a neon bulb.
I used to use a NE-2 bulb with 2- 100k resistors. One side of the neon bulb goes to Neutral. The other side of the neon bulb goes to one side of both 100k resistors. The other side of each resistor goes to the two switch legs.
The circuit makes use of the relatively high threshold voltage of the NE-2 bulb. This about 65 Volts AC.
When the switches are in a position that does NOT turn the lamp on, one resistor has 120 Vac applied, the other resistor is near 0 Vac. The neon bulb is NOT lit because the voltage across the bulb is less than the threshold voltage.
When the switches are in a position that causes the lamp to be lit, one of the 100k resistors is at 120 Vac, the other resistor is floating (not connected to anything). The neon bulb then lights.
You can place this circuit at each switch location. I used to drill a 1/4" hole in the cover plate and use silicone sealant to glue the neon bulb laying across the rear of the hole.
Note that this circuit also works with 4-way switches.
EDIT BY RYAN: added a schematic... Does this correctly show what you were discussing?
[Dwayne] Yes - exactly. In older houses without a neutral wire in the box, we would use appropriate voltage-rated resistors, then connect the Neutral side of the neon bulb to metal box so as to use Earth ground instead of Neutral. The Electrical Inspectors were okay with that because the current was so low (about 0.5 mA).
simulate this circuit – Schematic created using CircuitLab