It doesn't need to be registered/recorded to be valid.
Its best to contact the closing attorney before the closing date and let him/her approve it. I don't think it should be a problem, but its not something to bring up at the last minute.
The POA does not have to be filed - prior to the closing.
However, you must obtain permission from the lender to close with a POA.
Therefore, you should have your attorney get your POA pre-approved prior to the date of closing.
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