Nope. My Sydney-based Star Trek club heard the rumour and reported it during the making of the movie. We really weren't sure we believed it, but then Paul Winfield came to Sydney to make a movie called "On the Run"! I got to interview him about ST II - quite some time before the ST film was released - and asked him about the controversy and he confirmed it. And was stunned we'd dug up the rumour!
Perhaps the unseen portrait would have shown Khan, Marla and young Joachim?
Nope. I have it from Judson Scott himself that the portrait was of the child, and not his character.
However, Judson Scott and Ricardo Montalban had rehearsed their scenes (at Montalban's house) on the pretext that they were playing father and son. They got to the set and realised they were not related after all.
That could be true, but I doubt it -- the Extras Guild would have been taking work away from their members by granting Gene permission.
The costumes had already been made. Thus, when GR did put in his requests to fill them all, the SEG was
unable to supply his quota for those dimensions on the two days specified, and so he was able to request male and female fan extras of certain (tall) heights and weights.
The only four much shorter females cast were Bjo Trimble, Susan Sackett, office secretary Michelle, and Leonard Nimoy's assistant, Louise Stange, who wore specially made small white jumpsuits.
And I did see references that the SEG changed their rules after TMP to protect their membership from losing future jobs.
I really would like a source on these claims as I've heard actors tell these things, but I've yet to see them substantiated.
"The Making of Star Trek: The Motion Picture" by Susan Sackett (Pocket 1980), also Susan Sackett's monthly "Starlog" column, and "On the Good Ship Enterprise" (Donning) by Bjo Trimble.