I searched on every forum on internet and eventually end up fixing this by jumping in late here, but this is a great discussion! The scripting solutions from @d-healey and @Felix W. look like the cleanest way to handle the numeric display logic in HISE.
Once you get those meters working perfectly, it's always fun (and good practice!) to compare your plugin's metering against a real-world reference. If anyone needs a super quick way to check their room's SPL or monitor levels, you can just use a simple online tool like noisemeter.co. It's obviously not for high-precision master bus work, but handy for a quick sanity check against your final output!