Just to give you guys feedback on this. I'm having the same problem. I waited a while till my website in Webflow was 'finished', then did the PHP templating (ripping pieces apart, header/footer includes, and database support etc). Afterwards my client requested some changes and I had to edit in Webflow and re-exporting up to somewhere like 10 times. I could have done it by hand but prefered Webflow for the responsive features.
Every time I would need to rename my files to _webflow.scss and _mysite.scss, bypassing the annoying OS X dialog everytime (Are you sure you want to change to .csss?). Because I use Sass to put them together (with Codekit as well) to get 1 style.css. I also had to pay attention where I would put dynamic content, as changes to the static version will needed to be copied to the dynamic version as well.
I understand it would be very hard to make Webflow support dynamic content. But if this process can be improved it would be very helpful already. The very challenging design problem would be: How do you make the app workflow so that changes back and forth (between phases design / development) will not be cumbersome.