Hello! I'm a long-time Mac user who is gradually trying to shift over to open-source software. The biggest reason I haven't shifted farther and faster than I have is that a big part of what I do is DTP. After a prolonged study, I've concluded that LyX is the closest thing on the "market." I am currently trying it out to see if it's right for me. I looked over Kathryn Anderson's write-up on LyX-as-DTP, and found it vaguely inspiring, but I don't have the patience or the background to do a lot of coding. So I'll be watching the progress of your GUI with interest.
But what I'm writing to talk about is not the GUI. In fact, it may be incompatible with it. I was noticing that LyX written largely in C++, but Kathryn's site made it appear that Python also plays a role. It has this in common with my current favorite (and job-critical) piece of open-source software, Zope. I also note that you are moving to an XML-based file format. Zope is also good with XML. So all this made me think that my job, which is very much like Kathryn's hobby, would be vastly simplified if there were a server-side DTP program (or something resembling DTP) with a web interface. We could drop in the text, drop in the graphics, and let the end-user download the finished product. There could even be an option to tweak the code. Perhaps this is over-reaching. I don't think so, though. There is already ZPDF, a Zope product meant to do just this. It doesn't work very well yet, but it could serve as a model. Steve