Hey, A quick search of the internet confirms my hunch - off the shelf CO2 meters are mostly geared towards industrial applications - so there dont seem to be many cheap homeowner models. I guess most people are not digging deep pits in their backyards and fermenting things in them - how boring. It does look like you can get something for 100-200$ - I would personally feel like my life was worth that much… then again, if you wanted to spend a few hundred bucks, you could have made yourself a beer fridge and brewed your lager in the garage without all that digging!
I was going to ask if you knew anyone who was smothered in one of these cellars - and so clearly this is a dangerous situation. It seems to me that the idea of using an open flame to detect pockets of CO2 is a recipe for disaster, as my (very hasty) research suggests that a flame will stay lit well beyond the safe range for you to breathe.
Clearly this type of cellar is in use, so it should be possible to do it safely. I think your idea of venting the CO2 from the fermentation lock would be a good one - as well as an active ventilation system pulling air from the lowest point and discharging it far enough away from the opening to ensure the heavier CO2 doesnt sneak back in.
From a design standpoint you could probably reduce the risk somewhat by ensuring that your floors all slope to a single point, and then put your exhaust fan there.
Also, it seems like it would make sense to crunch some numbers. Like, how much CO2 is produced by fermenting 20L of lager? Roughly how many cubic meters of air do you have? How long would it take your fan to completely refresh that air? Using those figures, you should be able to come up with a relatively good idea of what would lead to an unsafe concentration of CO2 - and then dont do that.
One last thought I had - you are on a hill, right? How far sideways would you have to go to get to daylight? I am guessing far enough to not be worth trying to dig a ventilation shaft, but thought I would at least float the idea.