Hi,
I think you can use price ACL and affect a group to all users who have placed orders after the cut-off date.
Thanks to Acymailing, you would be able to send a newsletter to a filtered list of users and insert in the email content a HikaShop coupon or an HikaSerial serial.
The other solution is to add the new product to all selected orders. In order to do it, the best way would be to use SQL query (using PhpMyAdmin).
So there are several solutions and I can't tell you which one is the best. It would depend on you.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.