The
Wiktionary definition is accompanied by usage examples (click on the "Quotations" link at the end of the definition to see them) from a surprisingly long history. For example, George Bernard Shaw in 1897:
I have only seen the performance once; and I would not unsee it again if I could; but none the less I am a broken man after it.
So I think the word ought to be allowed, and for the sake of consistency, the past tense
unsaw.
Unseen and
unseeing are already allowed, for their use as adjectives.
Unsee is an unusual word, in that it seems to be almost always used negatively: something that isn't done, or cannot be done.