In terms of gamut, CMYK is completely subsumed by RGB: there are no colors that are in CMYK that are not in RGB, so converting to RGB is less destructive. I have done a lot of work in print that uses photographs of artifacts and paintings where the goal is to color-match the printed version to the actual object. We then use the imagery online.
For me, I have found that for the photography, working in CMYK as early as possible allows me to get to the color right without worrying about a destructive gamut change from RGB to CMYK. I shoot in RAW, run my preset curves, open in RGB and immediately convert to CMYK before even looking at the object for comparison.
If you are working with photography and have reasonable monitor calibration for CMYK work, then if it looks good in print and on screen, you will be able to preserve that look in RGB with little to no tinkering. If you have some you want to boost saturation (etc), you can relink those to an RGB version.
In the case of inDesign and any program that stores a swatch index (instead of a color) on an object, take your list of swatches (e.g. brand-affirmed list of colors) and make two sets, one for CMYK, one for RGB.
If your program uses the names given, you can probably overwrite one set with the other by simply using the same names. If not, you can probably remove one swatch and replace all instances of it with another of your choosing. In this example you'd remove the brandBlueCMYK swatch and assign brandBlueRGB as its replacement. Because you are working with a swatch, this change will propagate through the whole document.
The photos will be unaffected, and you now have a quick way of altering the document for RGB output that preserves your photography color adjustments.
For photos, if you make cmky and rgb folders and then use the exact same names in both branches, then relinking from CMYK to RGB is as simple as renaming the CMYK parent folder, opening the document, then re-freshing the links by pointing to the RGB folder.