Jeff... The system that stops your battery getting more than 15 volts should already be in place if you have a regulator fitted..
It's up to the operator to stop the alternator spinning if you get a red light. (assuming you have fitted a warning light.) Or cut the cables coming from the battery, if the problem is coming from that direction, as was the case for Nasher.
I am as electrically adept as yourself, but I fancy that it is not possible to monitor the battery level when the engine is running, as it will be receiving a charge current, and therefor that will be the current available anyway regardless of the battery.
If you have another look at Alan's BSM (Battery Status Monitor) which he has been making for over 20 years apparently. You will see that it works in both charging and battery level modes simply depending on whether the engine is running or not.. Something which the Gammatronix does not, without manipulation.
Alan's BSM also has the cute feature of dimming when the ambient light level drops.. ie at night. I think it's un-ambiguous red flashing when over charging is good too.
Anyway as mentioned I will be seeing him on Wednesday, so I could pick up a handlebar clamp for you if you like and you could try one of my Gammatronix units...I think I will be going for one of Alan's BSMs to be honest.
Guess I'll be seeing you at the weekend?