That is yet to be determined I believe.
But yes, if it is found Essendon have breached their duty of care, he can walk for nothing, I think.
At what point would this be determined? Not by the ASADA trial I wouldn't think.
A likely scenario IMO is that a player will say to the club "I want out of my contract, you breached your duty of care to me." At which point, the club would be absolutely mad to contest it. They will lose, and look like total dicks in the process. Establishing a breach of duty of care isn't that hard. It's obvious they did. They injected players with unknown substances without understanding or communicating those risks, and potentially putting careers in jeopardy.
The narcissists at Essendon might very well be mad enough to contest it.
If a player wishes to activate this clause, the AFLPA will back them 100%.