Legal Voltmeter System?

So our team recently purchased a bunch of small voltmeters so we could make an array of Vex chargers to display the battery voltage which would let us check the voltages much faster - not needing to check each voltage with a seperate multimeter.
Our plan going about this is either to splice the small voltmeter with the Vex battery extension cable or solder on a male tamiya connector lead on one end and a female lead on the other so that one end will plug into the charger and another into the battery.
I was just wondering if it is legal to do this, and to see if it violates any rules (such as ‘tampering’ with the chargers).

I have seen a bunch of these. As long as the Vex charger is supplying the power, it should be OK. The Vex Charger is the thing you need to use versus aftermarket ones.