Interesting, but probably less workable then we might think.

First of all, my EQ for the home system is two-channel, whereas my EQ for the car is four-channel. Second, the EQ for the car is radically different. The home system EQ is nearly flat, whereas the car system has radical changes to compliment the treble and bass speakers.

There is an issue with digitally overdriving the outputs if you add up multiple changes. If, say, your car EQ had the treble increased on the front speakers, and then your per-song EQ also had the treble increased, it might go past the upper limit and sound awful. At best, it might give you a clipped EQ curve that doesn't produce the sound you expected.