Using an identifier for what should be a nonblock operation doesn't seem like a smart solution to me, let alone the fact that $dns already exists and is not meant to initiate a dns request.
More importantly though, if by "non-existent" you mean unregistered, you can't differentiate a "non-existent" (unregistered) domain from one that won't resolve with the DNS protocol.. you would need to WHOIS to get this information. A non-existent domain and a non-resolvable host will both return NXDOMAIN. Try it for yourself. And since mIRC doesn't (and probably will never) natively support the WHOIS protocol, I don't see this happening.
