David, you are correct that InDesign does not, by default, push CMYK through its color management system to “cross-render” it to the new CMYK. That is on purpose, as most CMYK images are already targeted to a particular output and shouldn’t be messed with. (For example, if I spec 50% cyan in a CMYK image, I don’t want InDesign messing with that… I want it to just pass that 50% through.)
However, you can force cmyk images to be color managed in a couple of ways. You’d have to tell InDesign to honor its embedded profile in Object > Image Color Settings (or on import). And you can also turn off the “Preserve Numbers” feature when creating your PDF. Again, most people should not have to do this, but it can be helpful in certain circumstances.
I talk about this in more detail in my Lynda.com course on color management in InDesign.
By the way, another option might be to export your INDD file as an RGB PDF and then rasterize that by opening it in Photoshop and doing the conversion to CMYK there (if you have to have CMYK). But of course, then you’ll lose vector art.