Dont the panel mounted surge supressors have an indicating led. I have not installed one in 4 or 5 years but thought they connected across the mains of the panel?
A current transformer wouldnt work without any current flow... There are ways of course i am just not sure what would be legal. You could safe it by running off the main lugs directly to a fuse block mounted in the panel with as short a feed (from the lug to the fuse block) wire as possible and safe. I would use a 240v light and run the circuit across both lugs instead of a 120v Light. You could use 2 120v bulbs in series.
I know i will probably get crap for a few things i said. Not saying i recomend it, just saying if i did it how i would. I would use 240v rather than screwing with neutrals and different feeds etc because i know how screwing with neutrals can cause wierd electrical issues, even though a 120v bulb likely would work. It is just how i would do it is all.
Although... After thinking going 240v route you cant use an ordinary 125v switch.
I would just make sure that you have at least one outlet that is hydro power only. I believe it may be legal to tap into the transfer switch and put a very small sub panel on the incoming hydro feed (can get ones with like 4 circuits) and have an outlet. You could plug something into it that would then tell you there's power.
could also use a non contact voltage detector at the meter where the power comes in. Check it when you know the power is out to make sure it wont pickup your generator somehow though. Those are not always that reliable but can come in handy for at least getting a general idea.
For my server stuff I actually have a 12vdc wall wart plugged into the non UPS side and at the end there is a relay that hooks up to my environmental monitoring device. When power goes out I get an email, and when it restores I also get an email. Did that with arduino. Could do something similar too but would need to be at the transfer switch before the hydro in.