Graphite reacts rather quickly in hydrogen at 3000K. At 1000°C, I don't know. With luck, it's slower than graphite with oxygen, which is very slow at 1000°C. You might ask a graphite manufacturer, as this information is rather banal.
If you suspect traces of oxygen in the argon, why shouldn't you let a solid scrub it, rather than hydrogen? A hot metal powder would do that and leave no gaseous products. Something like hot iron powder.
Depending on grain size, metal powders can be dangerous too.
In case the oxidizing impurity in argon is water vapour rather than oxygen, a metal will scrub it too, while hydrogen doesn't. And even if the oxidizing impurity is oxygen, the water produced by hydrogen reaction may react with graphite.