There is a precedence, and probably more than one. In the days before the 2012 season, Melbourne elevated Tom Couch from the Rookie list. He had a good reputation, and quite a few locked him into their SC teams. From memory, just like Lambert, he was emergency in Round 1, and in quite a few other rounds, until he debuted in Round 16!
There is a big plus, and a big minus to keeping Lambert in your team.
The big plus is, you don't have to restructure everything because you are short of dollars (assuming you are) to upgrade him, and his DPP link is useful.
The big minus is, you might miss an obvious, and potentially only downgrade target if he's already sitting in your team. If he's on the bubble in round 6 or 7, and looking good, those that don't have him grab him with glee in a downgrade of a fat cow, while your left scratching your head looking for an option.
There's no right answer right now. If you are happy to park him at M11, that's ok, if you can afford someone else, that obviously ok too. I think if he's your only Rookie missing, and it upsets your structure to replace him, just keep him.