Here are some ways to contact Bryce. I can not guarantee that Bryce will see or even read any of your letters because she's a busy person, but it's always worth a try!

Bryce Dallas Howard
c/o PMK/HBH
700 San Vicente Bulevard
Suite G 910
West Hollywood, CA 90069

Bryce Dallas Howard
c/o Innovative Artists
235 Park Avenue South, 7th Floor
New York, NY 10003
USA

According to fanmail.biz this is Bryce's mailing adress, but I'm not so sure if it is the correct one. Someone at the fanmail.biz boards used the second one and received a reply, you can see it here.