The only option they offer which doesn't involve an iframe comes with a warning, "It will NOT submit to our servers. Because of limited resources, we cannot provide any support for helping you connect these files to your backend." (I'd post the URL but it's in a logged-in area of the site).
In any case, it looks like it's best if I stick with what I've got if I want to maintain error display... I have functionality in the form that's not possible to add with CSS, I'm using jQuery to set up ajax form submission / error display, and XPertMailer sends my messages through a gmail account on the back end.
Thanks for the info though.