As per guide, only tape destination using "-tape /<node>/<tape>" is available. No external and internal(WAFL) destination is abailable in SMTape so far. If you would like such functionality, consider to open a case for enhancement request.
I have the same request. I have used the 'smtape' method to initialize a SnapMirror relationship using a NAS device which was CIFS shared. This however was using 7-mode. I would assume that the same can be said for Clustered OnTAP??