After a bit of a hunt I found this reference to the rhetorical use of metastasis in “Luciferous Logolepsy”:

phr. - casual mention of a subject as if it were unimportant; responding to a person's criticism or insult with a riposte that uses or plays on his or her words

And then rather more here:

http://humanities.byu.edu/rhetoric/Figures/M/metastasis.htm