I just did a gig where the client needed the source done in Inkscape, but needed CMYK output for submission.
What I did, IIRC:
1. Attached a color profile to the Inkscape doc
2. Made sure all the objects had a correct color in the 'CMS' tab on the 'Fill/Stroke' dialog.
3. Imported the SVG into Scribus (after converting text to paths).
4. Exported PDF v1.3 with 'output' set to 'Print' (instead of 'Screen'/Web').
I'm skipping a few steps here (eg, having to resize), but that's the gist of what I did. The colors matched when I double-checked the final PDF in Illustrator.
I my case, I had a bunch of small (11pt) text that *had* to be 100% K and not rich black (which is what I was getting from straight-up PDF export from Inkscape), which is why I had to go through the import/export step in Scribus.
HTH, Chris
PS - I agree in principle about being able to submit RGB files, but for certain edge cases (rich black, spot colors, pure CMY, etc), not having CMYK export is pure pain. And it seems like all of the publishers/printers (in the US at least) insist on CMYK files for some reason (which sucks, really).