Another potential option is to use a neon bulb or lamp. The common neon indicator that I used to use is the NE-2H - this has fairly-wide current capability and would be able to handle the current range of caused by the supply voltage changing from less than 100V up to 1000V.
The downside is that a neon indicator does not match your requirement of indicating down to 40 Vdc. The NE-2H extinguishes (after being lit) at about 60 Vdc.
NE-2 & NE-2H indicators are still readily available. There are also much larger neon bulbs and lamps but they may not be readily available any longer. But you can check.
Final downside of a neon indicator is that they do die after an extended time. You have to weigh the consequences of the indicator failing some time in the future. Do note that they fail "gracefully" - they don't fail completely at one time, but rather, degrade. You would use that degradation as an indication that the lamp needs to be replaced.