Domain "masking" isn't achieved within the DNS proto itself. To do this, basically, you need to stand-up a webserver that serves out a frameset to the actual domain. (GoDaddy does this for you under the hood.)
You could do this on App Engine by creating a separate module that serves the frameset and then using dispatch.yaml to route requests for
abc.com to this module that serves the iframe for
xyz.com.
That said, domain "masking" is super-yuchy and totally breaks bookmarking. Don't do it. ;)