Is this grammar okay?

Sometimes as instructors we hear, see, say things and think, 'Is that real?  Can I say that? Would anyone else say that?  There must be a grammar rule about that."

Here's the questions a fellow instructor said one of her students had written this sentence for future perfect: "I will have been done by 3:00."  She was questioning if that sentence was grammatically correct.  She knew you can say "I will have finished by 3:00" but the part that was throwing her off was the "been done."  The question was, does that "been" make it passive?

Another instructor said, No- that's not a good sentence.  I said, Yes, it's a good sentence.
Here's why:
You can say "I will be done."  'I' is the subject, 'will' is the future marker, 'be' is the main verb and 'done' is an adjective.  It is similar to "I will be happy."
Seems like the same should be said for "I will have been done by 3:00."  'I' is subject 'will is the future marker, 'have' + past participle of the main verb 'be' is perfect, 'done' is adjective 'by 3:00' is a prepositional phrase.

They both looked at me quite skeptically and walked away.

I said, "I would say it! (if I ever used future perfect.)