The best you can hope for is to have a legit license key registered to a particular site and then spoof that site in requests from the nulled version (so the nulled version looks to their servers like it is coming from the legit site with the legit key).
But it's easy for them to track usage like that, higher than average requests being the most obvious way - the more people use the nulled plugin and spoof the legit site, the more obvious it is that the license key is compromised. If the average user is making say 200 requests a day but a particular license is making 4000 requests a day that's a huge flag.
