Hi,
I'm not sure if this is possible. Most guest operations use GuestAuthentication data object to pass authentication data, and looking at its documentation at http://pubs.vmware.com/vsphere-6-5/index.jsp#com.vmware.wssdk.apiref.doc/vim.vm.guest.GuestAuthentic... I don't see a class that extends GuestAuthentication and provides support for SSH keys.
This is possible - using the SSH plug-in.
You must register the vRO public key with the guest first (IE: Have it already present in your template if possible).
Then, going forward you can use the SSH plug-in to copy file(s) from vRO to the guest OS via SCP using the SSH key as authentication, negating the need to keep providing a password. Unfortunately, it doesn't look like the library workflows are written to provide this particular transfer option with key based auth.. but it isn't that hard to setup.
Take a look at the SSHSession object using the API Explorer:
Establish a session using your key:
var mySSHSession = new SSHSession(host,user);
mySSHSession.connectWithIdentity("/var/lib/vco/app-server/conf/vco_key");
Transfer a file:
var result = mySSHSession.putFile(myLocalFile,remoteFilePath); // returns 0 if successful, -1 if error has occurred
// add logging/debugging as needed
// close out the session:
mySSHSession.disconnect();