InDesign Server - convert all assets to CMYK?

Question

We have an InDesign Server service that changes text, images, colours etc in documents according to user input. When done, it outputs either an INDD package (zipped directory) or a PDF.

One of our clients requires all output to be in CMYK, according to their own .icc colour profile. I can get IDS to attach the colour profile to the file no problem, like this:

// Set CMYK profile
if(inputs['cmyk_profile'] !== undefined && inputs['cmyk_profile']) {
  app.colorSettings.cmykPolicy = ColorSettingsPolicy.PRESERVE_EMBEDDED_PROFILES;
  document.cmykPolicy = ColorSettingsPolicy.PRESERVE_EMBEDDED_PROFILES;
  document.cmykProfile = inputs['cmyk_profile'];
  //document.printPreferences.colorOutput = ColorOutputModes.COMPOSITE_CMYK;
}

I confess, though, that I am a bit of a newbie and the PRESERVE_EMBEDDED_PROFILES lines are only a vague guess of what I think should be happening. I also cannot get the last, commented-out, line to work because it throws the error: "Invalid parameter". According to the docs, this instance variable is "not valid when a device-independent PPD is specified" although I don't see why not.

My main problem is that the user-inputted images and colours can be in RGB format, and they need to be converted to CMYK, in that specific colour profile. (I have been told that simply adding the profile to the file isn't enough - I have to convert the assets as well)

I am also trying to enforce CMYK when converting to PDF too, like this:

// Set CMYK if document has it
if(document.cmykPolicy)
{
  app.pdfExportPreferences.pdfDestinationProfile = PDFProfileSelector.USE_DOCUMENT;
  app.pdfExportPreferences.pdfColorSpace = PDFColorSpace.CMYK;
}
else
{
  app.pdfExportPreferences.pdfColorSpace = PDFColorSpace.UNCHANGED_COLOR_SPACE;
}

// export pdf...
document.exportFile(ExportFormat.pdfType, myFile, preset);

But I have been told that the resulting PDF file is also RGB.

Is there any relatively painless way of doing this?