Iirc that's what they did at urban assault but with magazines rather than speedloaders as the majority of players were using M4 variants
Yup, my local CQB sites do that, loaded with 0.2g, with a maximum site weight of 0.25g, so it's reasonable on the face of it.
However, what it doesn't control for is when some clever-dickhead is intending to rock 0.4g in his HPA'd m4-mag Franken-Cappa, knowing that it'll chrono much lower with 0.2g and that the chances of being caught are essentially nil. I lost a fingernail to what I'm convinced was that scenario.
There's no perfect solution, but if everyone was informed and honest, then we wouldn't
need chrono. If you're going to do it, and do it for safety reasons, you might as well do it properly, and on the precautionary principle.