Not exactly what you are asking. But just in case...
You can easily authorize a specific ssh key to only be allowed to run certain commands on the remote machine (this is similar to how gitosis and gitolite and possibly any other secure git-over-ssh implementations work). Long story short, add these options to the destination users's
The file should look like this in the end:
command="/path/to/validation/script",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty ssh-rsa AAAAB3NzaC1yc2E.....
no-* options are self explanatory.
command= option runs a "forced" command "ignoring any command supplied by the client".
The command originally supplied by the client is available in the
SSH_ORIGINAL_COMMAND environment variable" (somewhat quoted from the
sshd man page). In other words when someone logs in with that key, the only command that will be run is the validation script. It is up to you to allow them to run it or not. So, your validation script can do something like:
if [[ $SSH_ORIGINAL_COMMAND == rsync --server -vtr --delete /source/path/ /destination/path/ ]]; then
And due to the other
no- options, this key is not allowed to do anything else too harmful.
Update: There is a ready-made script that simplifies this configuration a little. -- Mikko