A.M.
I like your idea. I know you said it's a pretty finished idea, but I have another one. When my husbands grandmother was living, for her 85th birthday, her 9 children and 36 grandchildren along with spouses did a memory "tree". Since it's christmas time maybe you could put them on a small fake tree or even small real tree like ornaments. Also, when she passed away the following year, one of her daughters copied all the memories that had been written along with old recipes of hers and put them in a scrapbook. Then she made color copies for everyone. Just thought I'd share!
Oh, and one more thing...Grammy used to make mittens every year for her children and then grandchildren. So, all the memories were written on mitten shaped cut outs. Maybe there is a special type/shape of cut out you could use to have all your family write their memories on?