I like the idea of having people write something for her.
I live abroad and all of my good friends are scattered across the globe, so I couldn't have a regular blessing way....but I did have them send a bead in the mail, along with a written message for me. I have to say it was really special to have those cards to keep and look at and remember the blessings. We also did something similar at our wedding party, had each person write a message for us folded on a small piece of paper and put in a box, and when DH and I are in a rough spot we take out one of the messages and read it. I reckon it might be really special for Brooke to have those blessings to look at later in her life. Perhaps you could ask people to write their best little piece of wisdom about life or something like that, that she'll read later on.
You could also give each person some seeds and ask them to plant a flower/tree/plant for Brooke with a blessing as they do, so there'll be plants growing in various gardens/woods for her.
That's all I could think of right now, not much I know. GL.