There would still be a draft, and it would simply to the benefit of the league, not the players.
The "league" would require a draft, and the league could set maximum limits on salaries for rookie players, effectively removing "all" power from the agents.
If the league puts forth a rule stating that all 1st round players will make 250K per year for their first 3 years in the league, and that the team drafting said player has their rights for their first three years in the league, there is nothing an agent can do about it, and the only thing a player could do is go to another league.
In addition to that, the "league" could impose an immediate rule for "blood testing" for steroids, HGH, and such. The player has only 2 choice here. Submit to the testing, or not play in the league.