Tal,
Like I said, I do not know how to do what you want, but have you looked at websitecm's mod?
http://www.websitecm.com/x-cart-mods/multiple-product-templates-package-for-xcart.html
From what I gather, it allows a different template to be used per category. I have never used it before, but maybe it could also work for fields.
Hope it helps.