Imagemagick will convert a bitmap file from RGB to CMYK color space. So will Scribus (with the help of an icc profile of course.) So I can reverse the process and take the CMYK factors, key them into a color mapping part of Scribus or Gimp, and jot down the RGB equivalent. The resulting palette will be an RGB palette that is within the CMYK gamut.
I just want to avoid all that labor.
You'll want to avoid more than just the labor. Depending on which color profiles and rendering intents you use for converting RGB to CMYK and back. the loss of precision could be huge. Doing that you'll loose even in-gamut colors, since the conversion isn't bidirectional and all the color values are likely to be affected. My suggestion is to stay away of conversions, and only use gamut warnings to know which colors are printable and which aren't, so you can create a color map containing untouched RGB values that are printable in the specified target CMYK space. Doing two conversions you'll get a map of clipped or shifted values that have nothing to do with your original map. And the plan is to keep only the printable colors out of an existing color map, right?