hi bowser
welcome to scribus.
i don't think that currently there are ways to solve it, except to split the file...
well, there is a "hack":
- create two directories, next to each other, for your images
- call them "images" and "images-high-res"
- when you add an image, add it first to the "images" directory, link it from scribus, move it to the high-res directory and copy back to "images" a low-res copy of your image
- when your finished (or when you want to output a high-res pdf), just close your document, rename "images" to "images-low-res" and then "images-high-res" to "images" and reopen the document.
of course, you will then have a very slow scribus (you won't even be sure to have enough RAM to open your document... so test it from times to time) but it's only for creating the PDF, so it might be ok.
all that being said, it's probably better to split your document...