ForwardAgent Yes
Defaults env_keep=SSH_AUTH_SOCK
I know of no existing way to do this.
sux is a tool that hijacks the $TERM variable to transfer additional data across the su barrier, in particular the $DISPLAY and xauth information. It could be modified to pass $SSH_AUTH_SOCK as well.
# make ssh-agent work inside Screen windows [ -z "$STY" -a -n "$SSH_AUTH_SOCK" ] && ln -sf $SSH_AUTH_SOCK ~/.ssh/ssh_auth_sock [ -n "$STY" ] && export SSH_AUTH_SOCK=~/.ssh/ssh_auth_sock