Hi there.
An one day registration is needed only for the dealer to get a cheaper price from factory by selling ONLY in D (not EU), because later it's not a new car anymore.
So just 1 day registration makes the car around 20 % cheaper than a blank reg.
mobile.de or autoscout.de are loaded with offers one day reg "Tageszulassung" .It ain't magic...
And think about the theme: VAT - makes a great difference.
Costs transporting car D to PL: 200/300 EUR.
Best would be, You talk with Your Leasing-Partner - they do this kind'a business every day
and with a lot of experience. Pay a little more, but You're on the good, safer way !
Czesc, Olaf