How accepted is proxies for tournaments?

So I could just get a normal daemon prince model for my death guard, however I feel it doesn't look very nurgley and I was considering proxying it with a harbinger of decay from AoS. It's the same size as the daemon prince, and I would ofc put it on the right base size.

But I'm wondering if that's just not gonna be allowed at any tournaments ever? I imagine it wouldn't work for most of them, which is a bit discouraging (even though I don't really play tournaments much lol)