“Here is a translation of another related teshuva from Rav Elyashev (Kovetz Teshuvos vol. 1 #174):
http://www.theyeshivaworld.com/coffeeroom/topic/men-withholding-a-get#post-566490″
That translation’s been posted all over the internet. It talks about the fact that we aren’t generally kofin in a case of ma’us alai. It has nothing to do with what we’re talking about.
If you have a link to the original teshuva by R. Elyashiv on the prenup that would be great. In Hebrew.