Within the world of the joke, scrod is a past participle in a passive construction, pure and simple. (woooo...)
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!
*The 'real pluperfect' in the context of the joke is "had scrod".
Thanks to some guy named CJ
