sftp-server only in subdir possible?

[DI Roman Fiedler]

I'm searching for a simple solution to allow access to only one 
directory of an existing user (that may not login) via sftp-server and 
authorized_keys file using the extended syntax

command="/usr/lib/openssh/sftp-server --root 
/data/exchange",no-port-forwarding,no-agent-forwarding,no-X11-forwarding,no-pty 
ssh-rsa AAA...keydata

Is something like that already possible, is there a better solution?

If not, how much effort for me would it be to implement it? I've already 
got the ubuntu sources and looked into the code a little bit. Is the 
chroot syscall after opening the root directory sufficient to make it 
work? Never programmed with chroot. Could such patch make it into the trunk?