G meters are certainly fitted to civil aerobatic-certified aircraft
The only certified aerobatic plane I've flown is the C150 Aerobat, and it is fitted with a G meter in accordance with the parts catalog for the plane - it's required equipment, as are the jettisonable doors and other features unique to the aerobatic version. I'm confident that other certified aerobatic aircraft are also equipped with a G meter.
you've been flying some strange loops and steep turns if the gz is similar. A balanced steep turn with 60 degrees of bank will generate about 2g; loop entries and exits will be just under 4g.
The 150 Aerobat can loop within 3G, if you're light (solo, half tanks). I would agree that a 3G turn would be "strange" in non aerobatic civil flying.
cocked-up aeros don't only lead to gz overstress.
Very true. The greater risk will be overspeeding the plane during a dive recovery, and there is no recording pointer on an airspeed indicator. A risk of aerobatics is that you are deliberately placing the plane such that without rapid corrective action (completing/recovering the maneuver) the plane will exceed a speed or G limitation. Thus entering the maneuver is one thing, but the skill is completing it within limits. When I have required to fly certification spin testing on a modified Cessna Caravans, I equipped the plane with a G meter on the glare shield. Happily, for the second program, based upon my protests, the developed spins were removed from the requirement. My protesting was based on my first program, during which I found it necessary to pull as much as 2.8G near Vne to recover the resulting dive. Had I not installed a G meter, and referenced it during the dive, I would have certainly oversped the plane, while pulling G - a very bad combination. As it was, the testing was successful, and I could confidently state that I had not exceeded a limitation in the client's plane! Airplanes are different in how quickly they speed up in a dive (and risk overspeed). Cessna Caravan, 210, 182RG, and Piper Tomahawk accelerate power off unusually quickly. Most float equipped planes I have flown are difficult to get to Vne at all (so it's usually reduced for float installations).
One general tool to protect innocent people from accidents caused by pilots is the PASSENGER BRIEFING.
In principle, yes. And I greatly endorse the important of a passenger briefing. However, many passengers I know may not be assertive enough to say "stop" when foolishness begins. And, many passengers may not recognize the approach to "maneuvering" until they're in it.... The key is a well disciplined pilot, who will fly with care and responsibility whether watched or not, rather than an adrenaline junkie!