Yes, you
CAN print it on the hang tag, but it must also be on something that is permanently attached to the shirt. Hang tags are meant to be taken off.
Here's a quote from the FTC site:
"Labels must be attached permanently and securely and be legible during the useful life of the product."
Here's the link to their page about complying with care labeling rules:
http://www.ftc.gov/bcp/conline/pubs/...h%20the%20Rule