I don't get the part about not having Diana. As far as I'm aware they is no Wonder Woman embargo...the live action women thing is a recent policy decision by DC, one I don't agree with. There's no reason Diana couldn't have been on the show. I would have written a two parter and in my Alternate Smallville that happens in the final season.
For a long time, as I understand it, there was a clause in some licensing contract saying that Wonder Woman could only appear in something where she was a main character. So she could show up in
Justice League (and JL-centric DVD movies like
The New Frontier and
Crisis on Two Earths) because she was one of the core regulars, and she could appear in a DVD movie in which she was the star, but she couldn't appear in a guest role in someone else's show, which is why she was absent from
The Batman and
Batman: The Brave and the Bold as well as
Smallville. I think the intent was to try to boost her profile by making sure she always appeared in major roles, but it had the opposite effect by diminishing her number of appearances overall. A prominent guest role can do a lot for a character's visibility, as we've seen with Green Arrow.
However, this policy no longer seems to be in effect, since Wonder Woman appeared in a supporting role in
Superman/Batman: Apocalypse and I gather she'll be appearing in a third-season TB&TB episode. So if
Smallville wanted to, they probably could bring Diana in. I doubt they'll have room in the time they have left, though.