The contents of the special present with proof of purchase of the complete Higurashi DVD series was announced on the
official site which is:
A DVD of the anime version of
Higurashi no Naku Koro ni Gaiden Nekogoroshi-hen.
The
Nekogoroshi-hen (cat slaughter chapter) was originally a short story by Ryuukishi 07 conceived as a present for readers who purchased the SquareEnix tankoubon manga for Volume 1 of Onikakushi, Watanagashi, and Tatarigoroshi respectively. This is a gaiden (side story) about Keiichi & Co.'s daily lives.
This Nekogoroshi DVD can be redeemed by entering with proof of purchase stamps for all 9 Higurashi DVDs (I do not know if the DVD set on sale December will contain the application stamp). Although all applicants can receive this, I do not know if they will ship internationally, as postage fee is normally required.
Since Vol. 9 won't be out until April, and it usually takes months until the merchandise is shipped out, this will be a long wait (I received a special Pani Poni Dash! guidebook 5 months after sending out the redemption procedures).
I stopped buying the Higurashi DVDs aftger Vol. 3, but this is good motivation for me to resume.