I used to run old electronic items that had a live or half mains voltage chassis from an isolation transformer with the chassis locally earthed independently. OK I know all about the dangers, but I'd suggest they are much less than running the devices on raw direct mains with no earth at all if you are leaving it open at the rear. So without quoting the really obvious pitfalls and breaches of rules in this, would you also take the chassis to an earthing rod, or to the earth on the primary side of the transformer? I don't see any detriment to the mains either way, since the final supply is isolated and you should be able to earth anything you like as long as the transformer is good and can isolate as it says on the tin, so to speak, it certainly has the current capacity, as its bloody heavy!
Brian