As part of his re-patriation agreement, he should require cameras to be rolling throughout the entire trial with a live uncensored feed available to any organization that wants it (News organizations, EFF, ACLU, etc). If the government shuts down the cameras for any reason, then the agreement is null and void and the USA guarantees his return to Russia.
Then the american people can decide if the trial is "fair" -- if the government tries to redact all of the evidence due to national security reasons, then it's hard to see how the trial can be called "fair".
I realize that the USA will likely ignore the agreement once he's on american soil, but at least it demonstrates that the USA government can't be trusted to abide by its own agreements and it validates Snowden's reason for fleeing to Russia.