I'm a bit closer to a solution here, but not within Scribus.
First of, it seems that since I was doing downscaling in Scribus to 300dpi (which should have been 600dpi anyway), the images were being anti-aliased. The anti-aliasing was then getting made into raster during the printing, making the lines look fuzzy or dotted.
I think I've now found a somewhat convoluted solution using Ghostscript, that seems to work (won't get a new proof print for a week, but zoomed in in Acrobat Reader with all image smoothing off, it looks correct). It's basically three steps:
1. Export from Scribus as PDF at full resolution (most of the drawings are between 600dpi and 1200dpi).
2. Convert to PostScript file at full resolution, but using monochromatic 1 bit images.
pdf2ps -dAntiAliasMonoImages=false -dDownsampleMonoImages=false -dMonoImageDepth=1 original.pdf intermediary.ps
3. Convert to pdf while downscaling the images using bilinear filtering.
ps2pdf -dAntiAliasMonoImages=false -dDownsampleMonoImages=true -dMonoImageDepth=1 -dMonoImageDownsampleThreshold=1 -dMonoImageResolution=600 -dMonoImageDownsampleType=2 -dDownsampleGrayImages=true -dGrayImageDepth=1 -dGrayImageDownsampleThreshold=1 -dGrayImageDownsampleType=2 -dGrayImageResolution=600 -dDownsampleColorImages=true -dColorImageDepth=1 -dColorImageDownsampleThreshold=1 -dColorImageDownsampleType=2 -dColorImageResolution=600 intermediate.ps final.pdf
Note, that this would only work for 1bit monochromatic images (line art), and would probably mess up your photos.
While the result isn't 100% identical, as the images are being scaled down, the bilinear filtering (DownsampleType=2) seems to do an ok job of it.
If anyone has a better solution, then please let me know. The exporting and conversion steps actually takes quite some time, so I wouldn't mind having a way to speed reduce it to just one step.
