¿QUE? You want to keep your cake or eat it?
If you are in Acrobat Pro you can choose to convert text or objects to CMYK and let RGB bitmaps stay in RGB, but since a design may mix bitmap and objects that is a risky workflow. What is it you are trying to achieve? If you use a PDF/x-4 workflow you are tagging your work in an output intent (Usually CMYK) and you can keep your RGB images RGB.
Are you talking of a "safe CMYK" workflow? Where CMYK is processed "preserve numbers"?
Really you will need to be more explicit if you want a more direct answer.