[Msquared]

Aside from any logistical objections to carrying around physical media, the one problem I see with this option is sound quality. Your portable CD player will have its own digital-to-analog (DAC) to get the signal to the aux jack in the console. Then, the radio unit most likely converts it back to digital to do whatever processing it does, and then finally sends it through another DAC process to get to the speaker outputs. Doing a full-res rip (not MP3) rip to a USB stick or phone and plugging that into the console port will eliminate an entire DAC process, including the removal of the poor analog output stage on your portable CD player.

One thought crossed my mind on this, though: what about a portable USB CD/DVD drive, like one would use with a laptop that doesn't have its own CD drive? That should stream a digital output to the car's USB port. The question is can the car's system recognize this is as storage like it does a phone or flash drive? I have no idea.