Completely open-source PDF/X export from Inkscape RGB via Scribus CMYK. It's really tedious, however it is possible.

First time

1. Open Scribus.
2. Close the "new document" wizard.
3. File→Preferences…
   1. Document→Page Size→Units→Millimetres (mm).
   2. Apply
   3. Document→Page Size→A6. (ie, If working with postcards)
   4. Document→Page Size→Margin Guides→Lock This is the little "broken chain" toggle button
   5. Document→Page Size→Margin Guides→Left: 0 mm
   6. Document→Page Size→Bleeds→Lock This is the little "broken chain" toggle button
   7. Document→Page Size→Bleeds→Left: 0 mm
   8. Colour Management→[X] Active Colour Management
   9. Preflight Verifier→PDF/X-3
   10. [OK]
4. File→Quit

Each time

In Inkscape

1. Open Inkscape
2. File→Open… {your document}
3. File→Save a Copy…→Name: something-something-curves-rgb.pdf
   1. Restrict to PDF version [PDF 1.4]
   2. Convert text to paths [X]
   3. Export area is page [X]
   4. [OK]
4. File→Quit

Command-line

1. Rotate if needed: pdftk x.pdf cat 1E output x-rotated.pdf

In Scribus

1. Ctrl+Alt+t to open a command terminal
2. Type: scribus something-something-curves-rgb.pdf (this is the only way to get the page auto-centred/aligned upon import without Scribus wanting manual placement)
3. Your document will probably look ugly and have the wrong colours, like ugly grey mud, this is Scribus' Colour Management mode. You can toggle it on and off with the small blocky icon on near to the right-hand side of the Scribus status bar at the bottom of the screen. However it must be on when you do the actual PDF/X-3 export.
4. File→Document Settings…
   1. PDF Export→File Options→Compatibility→PDF/X-3
   2. PDF Export→Pre-Press→Bleed Settings→Top: 1 mm
   3. PDF Export→Pre-Press→Bleed Settings→Bottom: 1 mm
   4. PDF Export→Pre-Press→Bleed Settings→Left: 1 mm
   5. PDF Export→Pre-Press→Bleed Settings→Right: 1 mm
   6. [OK]
5. Edit→Colours…
   1. Resize the window to be larger. You will want to see the full names of each colour. The 'Battenberg' icons are CMYK-defined colours. The 'vertical stripes' are RGB-defined colours.
   2. For each colour named "FromPS#nnnnnn", if it is black, or white, or a precise duplicate of another colour:
      1. [Delete]
      2. Replace With: [Black]
      3. [OK]
   3. For each colour named "FromPS#nnnnnn", that you need to map to its precise CMYK definition:
      1. [Edit]
      2. Name: {Corporate Pink}
      3. Colour Model: [CMYK]
      4. C: {nn} %
      5. M: {nn} %
      6. Y: {nn} %
      7. K: {nn} %
      8. [OK]
   4. [OK]
6. File→Document Settings→Colour Management→Activate Colour Management [X]other PDF/X-3 export will not be available
   1. [OK]
7. File→Export→Save as PDF…
   1. Output to File: {something-something-curves-cmyk.pdf}
   2. General→File Options→Compatibility: [PDF/X-3]
   3. Pre-Press→Bleed Settings→[ ] Use Document Bleeds
   4. Pre-Press→Bleed Settings→Top: {1 mm}
   5. Pre-Press→Bleed Settings→Bottom: {1 mm}
   6. Pre-Press→Bleed Settings→Left: {1 mm}
   7. Pre-Press→Bleed Settings→Right: {1 mm}
   8. Info String: {name and contact telephone number}
   9. [Save]