It's brilliant!
I think this is the best possible way to communicate the intended meaning. No words could be truer to the spirit of English grammar. How better to express the intended dishonesty of falsely backdating the record of an act to be done in the future than to "backdate" the normal future perfect have to a future pluperfect had? The logic is impeccable. "I will have done it" wouldn't just be weak, it would be wrong.
It's catachresis, that is, unconventional grammar used deliberately for effect. Now, a certain vocal minority of humanity is obsessed with the idea that grammar must work by strict, predictable rules, established somehow by authorities (whether "descriptively" or "prescriptively"), and it's their job to reject every nonstandard formulation as if they were inspectors at a meat-processing plant ensuring that not one E. coli bacterium contaminate the nation's food supply. They aren't going to like this.
"Nonsense!" they'll say. They'll cite federal regulations. They'll quote CGEL. They'll prove it's ungrammatical.
They're almost right. It's a nonce tense.
All language—all vocabulary and all grammar—works by a listener's ability and willingness to "play along" with the way a speaker has varied previous utterances to communicate something new. Some people won't get it. Some people will get it but won't play along. But many people will play along.
