Within the world of the joke,
scrod is a past participle in a passive construction, pure and simple.
It's not often I hear the past participle would be completely
lacking in humor because the past participle is used all the
time. The same would be true if you used
passive
in the
joke. That is why the joke has to mention some purely fictitious
or fantasy tense that sounds complicated (whether such a tense actually
exists or not). Therefore, from the point of view of the "tense"
to use in the punch line, there is no correct way to tell the
joke. Make up your own "tense" as you choose, preferably
something which just "sounds funny". Apparently the consensus is
that "pluperfect"* is an inherently funny word!
CJ
*The 'real pluperfect' in the context of the joke is "had scrod".